Upgrade to v5 elastic agents api

[GaneshSPatil]

See gocd/gocd#5937. V5 adds support for multiple clusters. We hope to get the v5 support in by 19.3 .

The idea is that an elastic profile will be associated with a cluster configuration (which is another key-value store, very much like an elastic profile).

You maybe able to follow the changes here: https://github.com/gocd-contrib/docker-elastic-agents/commits/master, or ask on the usual communication channels if you have questions/feedback.

/cc @GaneshSPatil

[GaneshSPatil]

\cc: @sgran

[sgran]

I'm sorry, we're no longer maintaining this actively - we're moving to the kubernetes elastic agent plugin instead.