webadmin: Refactored base Model class probihition reason option

Changed the base ui model prohibition reason from a list of Strings which
was only used with a single value to a string
Also fixed places in which the prohibition reason was not displayed correctly
due to the reason being set only after the changability property change thus
setting the reason after the rendering of the element as disabled

Change-Id: Idda39fe051b2547c202dab9a0a1970779001b70e
Signed-off-by: Tal Nisan <tnisan@redhat.com>