Enable dib service for heat-dsvm-functional

This is required to build the image used for functional tests.

Enabling the dib service just checks out some diskimage-builder
related git repositories. The actual custom image building will
be triggered from a change in the heat repository such as
change I9d27664638de95e52bc954e1fa00299e6711de90, which won't
be approved until image building proves stable.

Change-Id: I917b9eeb0558216da9a97f6a8ce9328d88085c44
Merge "Enable dib service for heat-dsvm-functional"