Add support for multiple nodes

[martialblog]

This changes the --hostname flag into an array of strings so that it can be used multiple this.

It also removes the --tls and --port flag which are now included as a full URL in the --hostname.
And the CHECK_ELASTICSEARCH_HOSTNAME env variable is removed, since it was designed for a single hostname.

Some examples:

With multiple nodes:

go run main.go health -H http://localhost:9200 -H "http://localhost:9202"
[OK] - Cluster eventdb-cluster is green 
| status=0 nodes=3 data_nodes=3 active_primary_shards=0 active_shards=0

When one node is down (I have no indices so all is green, note the nodes=2):

go run main.go health -H http://localhost:9200 -H "http://localhost:9202"
[OK] - Cluster eventdb-cluster is green 
| status=0 nodes=2 data_nodes=2 active_primary_shards=0 active_shards=0

When no node is reachable:

go run main.go health -H http://localhost:123 -H "http://localhost:321"
[UNKNOWN] - could not fetch cluster health: No node reachable (*fmt.wrapError)

[RincewindsHat]

I do like the functionality and in general I do approve of this.
My worry now is, that those changes WILL break existing setups if the old version is already in place

[martialblog]

Yes indeed, I will note that in the Release Notes and we should state it in the packages.

On the other side, I do think plugin is pretty feature complete after my recent changes and we could move to a 1.0.0