Add multi-SCM capabilities

Jenkins has a multiple-SCM option. We can just support that by using it when
more than one SCM module is defined. Simple enough.

Change-Id: Iacfba5c547ff111304644a4777a12a70b0b22e62
Reviewed-on: https://review.openstack.org/23961
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Antoine Musso <hashar@free.fr>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins