etcd: compatibility with check mode

[vitabaks]

do not install etcd in check mode

ansible-playbook --check (don't make any changes; instead, try to predict some of the changes that may occur)

Fixed:

TASK [etcd : Download "etcd" package] ******************************************
failed: [10.129.50.35] (item=https://github.com/etcd-io/etcd/releases/download/vv3.3.27/etcd-v3.3.27-linux-amd64.tar.gz) => {"ansible_loop_var": "item", "changed": false, "dest": "/tmp/", "elapsed": 0, "gid": 0, "group": "root", "item": "https://github.com/etcd-io/etcd/releases/download/v3.3.27/etcd-v3.3.27-linux-amd64.tar.gz", "mode": "01777", "msg": "Request failed", "owner": "root", "response": "HTTP Error 404: Not Found", "size": 4096, "state": "directory", "status_code": 404, "uid": 0, "url": "https://github.com/etcd-io/etcd/releases/download/v3.3.27/etcd-vv3.3.27-linux-amd64.tar.gz"}

Related PR: https://github.com/vitabaks/postgresql_cluster/pull/459

Additionally:

• Add ETCDCTL_API: "3" to environment
• expect 'is healthy' status in stdout or in stderr
  • In certain scenarios (for example the old etcd version), even when a command executes without errors (.rc=0), the output may reside in stderr instead of stdout. Now we accommodate for 'is healthy' in either output stream.