Add support for Ubuntu 18.04

Next Ubuntu LTS version is 18.04 and it will be released in April 2018. We should look into what is needed to create omnibus-gitlab packages.

Few things come to mind:

1. Omnibus needs to be able to recognise the platform correctly
2. Build images need to be created for this version
3. Package server needs to support this platform

cc @gitlab-build-team