core,restapi,webadmin: Added description field to storage domain

Added a description field to the storage domain entity, changes also made
to the webadmin and REST to reflect this change

Change-Id: I86bcd7c52e5d9cad7c3cb30561718203537ab459
Bug-Url: https://bugzilla.redhat.com/614859
Signed-off-by: Tal Nisan <tnisan@redhat.com>