Allow overcloud image customizations

Adds three new environment variables that allows you to
externally control how the overcloud-controller,
overcloud-compute, and overcloud-cinder-volume images
are constructed:

 - OVERCLOUD_CONTROL_DIB_ELEMENTS
 - OVERCLOUD_COMPUTE_DIB_ELEMENTS
 - OVERCLOUD_BLOCKSTORAGE_DIB_ELEMENTS

The motivation is to be able to build images that use Puppet
for configuration using devtest_overcloud.sh. This should help
facilitate future CI testing using Puppet w/ packages.

Change-Id: I2e004b04420839d2a887fa6f8534d5744f2a32c9