Fix git group detection for gitolite ssh user

The tasks gitlab:env:info mixes user and group, and presume as a group 'git'.
However, gitolite group name can be anything.

That patch add the git group name in the config,
and check gitolite.ssh_user group again git.group.

M config/gitlab.yml.example:
  Add 'group' in 'git' section
M lib/tasks/gitlab/check.rake:
  check that gitolite.ssh_user group is the one defined in git.group.
  Note: uses a more complete regexp for the group detection
        (the group can start, end or be in the middle or the list of groups
         of gitolite.ssh_user)