Enable elastic search plugins

Provide the elasticsearch module with a parameter to allow configuring
plugin location and then use it in the role class to point to plugins
deployed via git-deploy/sartoris. Plugins gerrit repository is at
operations/software/elasticsearch/plugins

Change-Id: Ib6722236530f3f7b7991258e98eba5da451d7612
Merge "Enable elastic search plugins" into production