Adding Build User Vars support

Adding support for the Build User Vars plugin.  This plugin
exposes new environment variables of who started a build, if it
was started manually.

Change-Id: I731419f26a9e59800ea2209d1c5a659a1b6cd483
Reviewed-on: https://review.openstack.org/30785
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins