Enable DIB service to build with minimal net access

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
Merge "Enable DIB service to build with minimal net access"