Allow gitlab-elasticsearch-indexer to have a fixed version

As discussed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2020#note_31924715 , we currently build gitlab-elasticsearch-indexer against master for all releases.

We'd like to merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2020 and https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/merge_requests/6 together in time for %9.4.

We cannot allow https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/merge_requests/6 to be backported to %9.3 or previous releases so before we can do that, we need to alter omnibus-gitlab so it builds specific tags of gitlab-elasticsearch-indexer.

I've just created a v0.1.0 tag that GitLab 9.3 and earlier can use. Once we resolve this issue, we can create v0.2.0 with the changes for %9.4.

@marin can you schedule this? I didn't have a complete answer in https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1455 :/