Isolate elasticsearch-using specs

Currently, we set up an elasticsearch service for all of our specs in GitLab EE. Instead, just as we do for specs that require the ~Geo tracking database, we should introduce a :elasticsearch tag and isolate these specs - and so the need for the elasticsearch service - from the main spec jobs.

Since the elasticsearch service is slow to start up, uses lots of RAM and sometimes fails, this should improve the reliability of the spec suite.