Refactor association controllers

[sebgl]

Kibana, APMServer and EnterpriseSearch have a very similar association logic with Elasticsearch.
There respective association controller code is pretty similar, but also diverges a little bit, which makes maintenance complicated in the long-term.

We could create a common esassociation package holding the logic to associate a generic resource to Elasticsearch.
Each individual association controller would still exist, but would rely a lot on the common esassociation library.

It also simplifies unit testing: most of the testing is done in a single place (the common package). Unit tests for individual controllers can be much simpler and focus on high-level testing and specific parts.