Add user support for git configuration

I tried to configure git for my vagrant user but it was impossible.
Instead the `.gitconfig` was saved for the root user:

    # Before provisioning
    root@webserver:/home/vagrant# ls -al ~
    total 24
    drwx------  3 root root 4096 Mar 12 17:35 .
    drwxr-xr-x 30 root root 4096 Mar 12 16:07 ..
    -rw-------  1 root root  289 Mar 12 17:45 .bash_history
    -rw-r--r--  1 root root 3106 Jul  3  2012 .bashrc
    drwxrwxr-x  3 root root 4096 Mar 12 16:07 .gem
    -rw-r--r--  1 root root  140 Jul  3  2012 .profile
    # After provisioning
    root@webserver:/home/vagrant# ls -al ~
    total 28
    drwx------  3 root root 4096 Mar 12 17:45 .
    drwxr-xr-x 30 root root 4096 Mar 12 16:07 ..
    -rw-------  1 root root  289 Mar 12 17:45 .bash_history
    -rw-r--r--  1 root root 3106 Jul  3  2012 .bashrc
    drwxrwxr-x  3 root root 4096 Mar 12 16:07 .gem
    -rw-r--r--  1 root root   94 Mar 12 17:45 .gitconfig
    -rw-r--r--  1 root root  140 Jul  3  2012 .profile

I used a similar configuration to [this one](https://github.com/czerasz/vagrant-presentation-vm/blob/master/provisioners/puppet/modules/profile/manifests/base.pp#L15-l23)

After the change I can set the config for a particular user:

    git::config { 'user.name':
      value   => 'Mike Color',
      require => Class['git'],
      user    => 'vagrant',
    }

    git::config { 'user.email':
      value   => 'email@example.com',
      require => Class['git'],
      user    => 'vagrant',
    }