Plugin should mask http passwords in saved metadata

[andrewvc]

HTTP Poller metadata will show passwords. They should be elided by default.

Example of current behavior:

"_http_poller_metadata": {
                  "name": "haproxy",
                  "host": "andrewvc-elastic.local",
                  "url": {
                     "method": "get",
                     "url": "http://localhost:9201/haproxy?stats;csv",
                     "auth": {
                        "user": "myuser",
                        "password": "mypass"
                     }
                  }

[abdalians]

+1