gitconfig::user to easily craft .gitconfig user files

The new module git contains a single define 'gitconfig::user' which let
one craft a .gitconfig file using a hash defining its content.  Simple
example is provided in the class documentation and a very basic rspec
test is provided to check whether the expansion happens properly.

Example usage:

  gitconfig::user { 'gitconf for jenkins user':
    homedir => '/var/lib/jenkins',
    settings => {
      'user' => {  # '[user]'
         'name'  => 'Antoine Musso',  # 'name = Antoine Musso'
         'email' => 'hashar@free.fr', # 'email = hashar@free.fr'
      },  # end of [user] section
    }
  }

Change-Id: I49a03405fbe715269e396f6f1e0c956ba10a5f0a
Merge "gitconfig::user to easily craft .gitconfig user files" into production