Add packer-verify-build jobs

Add packer-verify-build jobs

This job is made to be manually triggered in order to build the
packer image, so that the full build process can be validated before
merging. If this is done on the final patch that is merged, the
merge job will not run another build (thereby avoiding building two
identical images).

The verify-build job piggybacks on the merge template, since it is
essentially the same job. However, it only triggers when given the
keyword "verify-build" or "packer-build". Due to the similarities,
some documentation improvements have also been shifted over to the
Packer Merge docs, as well as some small changes to Packer Verify.

Issue: RELENG-2671
Change-Id: Id4fb33a88447bbb2f09ac0967ac303a8f6d62326
Signed-off-by: Eric Ball <eball@linuxfoundation.org>