webadmin,userportal: error in custom properties sheet

Current custom properties sheet doesn't handle cases
where there are no properties stored in DB.

Extracting the new duplicate code (splitting logic) into AsyncDataProvider

Change-Id: Ia939952c367b6416ed90bb295f1485ce4ac63368
Bug-Url: https://bugzilla.redhat.com/924201
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>