matrix jobs: Add slave axis support.

This commit builds on the structure introduced by
Ib2f36e10c1c5fb14f0e4255f35c0bf19c2cc39b3. Slave axis
has the same XML structure than other axes, only the
element name changes.

Change-Id: Ie1d2ef96c7a7d9aa02066ff055ead7c84011bdc6
Reviewed-on: https://review.openstack.org/27656
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