Adding new options to git checkout

A new option has been added to git checkout called name that
specifies the name to fetch.  Also added the ability to specify
the git tool to use by specifying git-tool: Default

Change-Id: Ie985c39fb5c6dc6ad84ee97bfb203da9fd689b8c
Reviewed-on: https://review.openstack.org/18817
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins