Ansible docker dependency

This MR adds elasticsearch hosted on docker, with docker added as a dependency.

Using docker will allow us to add further containerized services to any deploy, elasticsearch being the first.

Please test by putting following node in gitlab-vagrant.yml:

- name: gitlab-ee-latest
  version: 9.4.1
  edition: ee
  ip_address: 192.168.15.53
  elasticsearch: 5.3.1

Then run:

vagrant up

Now log onto http://192.168.15.53, create a root user and add the shared license.

Then enable elasticsearch using the URL that was printed to screen at the end of vagrant up.

Then the index command should from from the commandline:

sudo gitlab-rake gitlab:elastic:create_empty_index