Add the ability to set git user.email and user.name

It is possible the host system does not have git properly configured,
which results in merge failures because the git client is complain. For
example:

  GitCommandError: 'git merge FETCH_HEAD' returned exit status 128:
  *** Please tell me who you are.

  Run

    git config --global user.email "you@example.com"
    git config --global user.name "Your Name"

  to set your account's default identity.
  Omit --global to set the identity only in this repository.

Now we can pass user.name and user.email settings to git, if configured
to do so.

Change-Id: I896194d8d1f5334026954b02f3a1a8dd82bed2ac
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-on: https://review.openstack.org/29015
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins