Extract instance related logical units from cmdlib

All LUInstance* classes are extracted to instance.py. Common functions
are moved to common.py if used by non-instance logical units as well.
Additionally, helper functions which are only used by LUBackup* and
LUInstance* are moved to instance_utils.py.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>