This option allows to disable users from changing their username.

This is very usefull in environments using strong internal authentication methods like ldap, pam or shibboleth.

You can allow users to change theyr username in these environments, but then new users (users loging in first time) is blocked from gitlab is her username exists.

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)
M: config/initializers/1_settings.rb:
  Add default values for gitolite.group and gitlab.user

Merge branch 'master' into non-interactive-aws-install

Conflicts:
	doc/installation.md

Fix merge mess in installation.md

Closes #1237