Consult gitlab-elasticsearch-indexer version from gitlab-rails

When gitlab-elasticsearch-indexer was introduced as an ee-only component, we declined to give it a GITLAB_ELASTICSEARCH_INDEXER_VERSION file. However, we now need this component in the tests, as the Ruby indexer will be removed soon: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12680

To use it in the tests, we need to know which version to use. To do this, it's easiest to follow the existing, e.g., GITLAB_SHELL_VERSION file scheme, so we can take advantage of all the existing tooling around building it in development. So, omnibus has to start supporting this file, instead of being canonical for the version.

Ideally, we still won't build this component for the gitlab-ce packages.