Enforce default values for gitolite config

Make sure the two previous pull requests about
gitolite install path and git group are using default values
identical to what existed before.

I.e.: if not define by the user, those missing settings won't alter
the current rake behavior.

M config/gitlab.yml.example:
  Mention default value for the two extra settings
M lib/tasks/gitlab/check.rake:
  Make sure to default to 'git' as the expected group
  if said group is undefined in the config.