Add ability to build separate fs images

This patch is required to build images for every separate mountpoint needed
both for centos and ubuntu.
Eg. /boot, /var, /var/lib, etc.
File system support is limited to ext2, ext3, ext4 only.
No XFS support mainly because of unexpectly consumed free space and absence of
working resizing tools due to that.

Controlled by SEPARATE_IMAGES environment variable. String format should be
space separated list with comma separated mountpoint and filesystem type. Eg.:
SEPARATE_IMAGES="/boot,ext2 /,ext4 /var,ext3"
Defaults to "/boot,ext2 /,ext4". Notice, that at least separate /boot is
required for operating of image based provisioning.

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