webadmin: add PreparingForMaintenance to Enums files

Added StorageDomainStatus -> PreparingForMaintenance
to Enums java/properties files: for proper display in the UI.

Change-Id: I075fe55cc3e6f19e715b3742abfea91cd59f05cb
Signed-off-by: Daniel Erez <derez@redhat.com>