packaging: setup: reading VDSM supported cluster levels from file

VDSM can still be not ready/configured. Intead of getting the
supported cluster levels from API it's safer to directly
read them from a file.

Change-Id: I79c8a254cae27a918a6d2de9dc6b37867bd7f8ce
Bug-Url: https://bugzilla.redhat.com/1100236
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>