matrix jobs now supports user-defined axes

The XML definition of a user defined axes is the same as a a labeled
expression axis.  Only the XML element name change, adding support for
user defined axis is thus only a matter of adding a map between our YAML
values and Jenkins XML element name.

Change-Id: Ib2f36e10c1c5fb14f0e4255f35c0bf19c2cc39b3
Reviewed-on: https://review.openstack.org/25432
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins