Disks of ext template are allowed to have arbitrary parameters
stored in the Disk object's params slot. Those parameters can be
passed during creation of a new disk, either in LUInstanceCreate()
or in LUInsanceSetParams(). Still those parameters can not be
changed afterwards. With this patch we override this limitation.

Currently, for the other disk templates we allow modifying only
'name' and 'mode'. Therefore, we introduce new constants
MODIFIABLE_IDISK_PARAM* to include those params. If any other
parameter is passed, _VerifyDiskModification() will raise an
exception.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>