Add support for custom gradle version aka gradleName

Change-Id: Ib74bcf852e128a8dfc1a297c5b9dc2571e5666ff
Reviewed-on: https://review.openstack.org/27791
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