A service to install diskimage-builder

Enabling service 'dib' will install the following repos:
* diskimage-builder
* tripleo-image-elements
* os-collect-config
* os-refresh-config
* os-apply-config

These repos are already pre-fetched in devstack-gate.

This will facilitate gating on changes in these projects by building
a custom image then running the heat-slow job against that image.

diskimage_builder is pip installed from the current git checkout
if the dib service is enabled. This allows devstack gating on
diskimage-builder changes while also allowing diskimage-builder to
be installed from releases for other scenarios (for example, ironic).

Change-Id: Ia911cdee86f5b2e2ba1557e5aa8bf392b92ef555
Merge "A service to install diskimage-builder"