improve flag validate output

it print the whole usage currently, the exact useful first line is two screen above. Propose to do it like this

$ ./etcd -proxy on -listen-client-urls 127.0.0.1:8080 -initial-cluster "infra0=http://10.66.11.39:2380,infra1=http://10.66.11.70:2380,infra2=http://10.66.102.87:2380"
invalid value "127.0.0.1:8080" for flag -listen-client-urls: URL scheme must be http or https: 127.0.0.1:8080
See 'etcd -h | --help'

Original output

$ ./etcd -proxy on -listen-client-urls 127.0.0.1:8080 -initial-cluster "infra0=http://10.66.11.39:2380,infra1=http://10.66.11.70:2380,infra2=http://10.66.102.87:2380"
invalid value "127.0.0.1:8080" for flag -listen-client-urls: URL scheme must be http or https: 127.0.0.1:8080
usage: etcd [flags]
       start an etcd server

       etcd --version
       show the version of etcd

       etcd -h | --help
       show the help information about etcd

member flags:

    --name 'default'
        human-readable name for this member.
    --data-dir '${name}.etcd'
        path to the data directory.
    --snapshot-count '10000'
        number of committed transactions to trigger a snapshot to disk.
    --heartbeat-interval '100'
        time (in milliseconds) of a heartbeat interval.
    --election-timeout '1000'
        time (in milliseconds) for an election to timeout.
    --listen-peer-urls 'http://localhost:2380,http://localhost:7001'
        list of URLs to listen on for peer traffic.
    --listen-client-urls 'http://localhost:2379,http://localhost:4001'
        list of URLs to listen on for client traffic.
    -cors ''
        comma-separated whitelist of origins for CORS (cross-origin resource sharing).

clustering flags:

    --initial-advertise-peer-urls 'http://localhost:2380,http://localhost:7001'
        list of this member's peer URLs to advertise to the rest of the cluster.
    --initial-cluster 'default=http://localhost:2380,default=http://localhost:7001'
        initial cluster configuration for bootstrapping.
    --initial-cluster-state 'new'
        initial cluster state ('new' or 'existing').
    --initial-cluster-token 'etcd-cluster'
        initial cluster token for the etcd cluster during bootstrap.
    --advertise-client-urls 'http://localhost:2379,http://localhost:4001'
        list of this member's client URLs to advertise to the public.
        The client URLs advertised should be accessible to machines that talk to etcd cluster. etcd client libraries parse these URLs to connect to the cluster.
    --discovery ''
        discovery URL used to bootstrap the cluster.
    --discovery-fallback 'proxy'
        expected behavior ('exit' or 'proxy') when discovery services fails.
    --discovery-proxy ''
        HTTP proxy to use for traffic to discovery service.
    --discovery-srv ''
        dns srv domain used to bootstrap the cluster.

proxy flags:

    --proxy 'off'
        proxy mode setting ('off', 'readonly' or 'on').
    --proxy-failure-wait 5000
        time (in milliseconds) an endpoint will be held in a failed state.
    --proxy-refresh-interval 30000
        time (in milliseconds) of the endpoints refresh interval.
    --proxy-dial-timeout 1000
        time (in milliseconds) for a dial to timeout.
    --proxy-write-timeout 5000
        time (in milliseconds) for a write to timeout.
    --proxy-read-timeout 0
        time (in milliseconds) for a read to timeout.

security flags:

    --ca-file '' [DEPRECATED]
        path to the client server TLS CA file.
    --cert-file ''
        path to the client server TLS cert file.
    --key-file ''
        path to the client server TLS key file.
    --client-cert-auth 'false'
        enable client cert authentication.
    --trusted-ca-file ''
        path to the client server TLS trusted CA key file.
    --peer-ca-file '' [DEPRECATED]
        path to the peer server TLS CA file.
    --peer-cert-file ''
        path to the peer server TLS cert file.
    --peer-key-file ''
        path to the peer server TLS key file.
    --peer-client-cert-auth 'false'
        enable peer client cert authentication.
    --peer-trusted-ca-file ''
        path to the peer server TLS trusted CA file.

logging flags

    --debug 'false'
        enable debug-level logging for etcd.
    --log-package-levels ''
        set individual packages to various log levels (eg: 'etcdmain=CRITICAL,etcdserver=DEBUG')

unsafe flags:

Please be CAUTIOUS when using unsafe flags because it will break the guarantees
given by the consensus protocol.

    --force-new-cluster 'false'
        force to create a new one-member cluster.

etcd-io / etcd

improve flag validate output #3141