Introduce new Pithos variables

Introduce new Pithos variables for selectable storage backend.
Extend Ganeti create script

Include new Pithos variables for selectable storage backend to Ganeti
create script.
Extend pithcat for selectable storage
Add default values for Pithos backend and RADOS

Set default values for Pithos storage backend type, RADOS conf file,
RADOS map and block pools.
Update documentation
Merge branch 'feature-pithos-backend-storage' of https://github.com/cnanakos/snf-image into develop
docs: Add section to install snf-image on CentOS
docs: Update installation guide to use root prompt
Don't let sgdisk align when creating partitions

In order to extend a partition we need to remove and recreate it. If the
partition was misaligned then the image is to blame. snf-image is not
allowed to change the starting sector of the partition since this will
destroy the file system it hosts. In order to make sure this does never
happen we need to set the sector alignment multiple to 1.
docs: Update the sample images links

* Add (k)ubuntu 14.04 and remove 13.10
* Add Oracle Linux
* Remove Debian Squeeze
* Add OpenBSD 5.5 and remove 5.4
Update the AUTHORS list

Add Chrysostomos Nanakos to the list of snf-image authors