The components PithosBackend, Image, GTools, and GanetiCollectd
are related to Ganeti but used only by Synnefo.

Define new role, named 'snf-ganeti' to include those components and use
it with the `snf-deploy ganeti` command.

Install ganeti-instance-debootstrap so that we have a working os
provider.

snf-image takes a lot of time to be configured (curl,
snf-image-update-helper, wget debian-base image) and is not needed
for a simple Ganeti cluster (i.e. QA cluster).

Move wget of debian base image to Image component and not nfs.
In order not to download many times check if it already exists
in the proper path.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>