Adds two new options to lib/dib that will help devstack slaves
use cached content and content local to cloud provides:
* DIB_BUILD_OFFLINE: This will enable DIB to rely entirely on
cached images without making any additional calls to cloud-images.ubuntu.com
to validate hashes/freshness.
* DIB_APT_SOURCES: Used to specify alternatve sources.list for image builds.
Setting this enables the addition of the apt-sources element during image
builds and is only supported for ubuntu/debian builds.
Change-Id: I9b9ca72ec551565d454610aacb86c585f0384f13
Partial-bug: #1375488