Enable gating of diskimage-builder

Uses the standard OpenStack tox methods to run tests and build
artifacts. Diskimage-builder recently added support for all of them
except sphinx, as diskimage-builder does not have any sphinx docs.

Change-Id: I948ce91836edadf526b0846218854b47f4bb1c34
Reviewed-on: https://review.openstack.org/21190
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins
Make pep8 a canary build.

For check jobs, build pep8/pyflakes before trying to
build unittests. This should cut down on running tests
on stuff that isn't going to pass muster at all quickly.

For projects that use pyflakes, make that the canary.

Leave gate jobs alone, since the likely hood of rebase
introducing pep8 errors is pretty low.

Change-Id: I739f55d3fc5acb113cf5851a63ba1bcbfbf440f3
Reviewed-on: https://review.openstack.org/21267
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins