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 against git.group
(which defaults to 'git', as before this patch, if undefined).

M config/gitlab.yml.example:
  Add 'group' in 'git' section
  Mention default value for the two extra settings
M lib/tasks/gitlab/check.rake:
  Check that gitolite.ssh_user *group* is the one defined in git.group.
  Make sure to default to 'git' as the expected group
    if said group is undefined in the config.
  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)