Building OS images centos and ubuntu

Those images are supposed to be used for
image based provisioning. This patch set
needs python-imgcreate package to be available.
Manual workaround is used to make
separate images for / and /boot for centos.

Images are only to be built if experimental
feature group is enabled.

Implements: blueprint image-based-provisioning
Change-Id: I4f038c99d4caf7d2b6e291bff29572b3a45d92c7
Merge "Building OS images centos and ubuntu"