Disable the experimental GitLab elasticsearch indexer checkbox if it's not in $PATH

Before raising an issue to the GitLab Community Edition issue tracker, please read through our guide for finding help to determine the best place to post:

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1483 added a setting to application_settings that is broken if gitlab-elasticsearch-indexer is not in $PATH.

If omnibus-gitlab#2168 (closed) is not done in time for GitLab 9.1, then it will be a poor user experience. Install-from-source users will suffer too.

If gitlab-elasticsearch-indexer isn't in $PATH, we should disable the checkbox and force the use of the Ruby indexer. This will need to go into 9.1

/cc @DouweM