Run elasticsearch specs against earliest and latest supported version of elasticsearch

Per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2751#note_38420070

Currently, we have a specific range of supported elasticsearch versions, but we only run the specs against the latest supported version of elasticsearch.

Taking inspiration from our mysql support, if we're on master or a branch matching /elastic/i, let's add a job that runs the elasticsearch-using specs only against the earliest supported version.

We'll need to find and tag all specs using elasticsearch to make this happen, but that shouldn't be too hard.

The current supported version range is 5.1 - 5.5

/cc @vsizov @smcgivern