Add ability to build separate fs images for ubuntu

This ability required to build images for every
separate mountpoint needed.
Eg. /boot, /var, /var/lib, etc.
File system support is limited to ext2, ext3, ext4 only. No XFS.

Implements: blueprint image-based-provisioning
Change-Id: I650052154fe7b483aaba89389e9a09f80fc1543e