webadmin: Moved provider filtering to AsyncDataProvider

Filtering by provided entity type is now done in AsyncDataProvider, to
facilitate moving the filtering to the backend if we ever want to do
that.

Change-Id: I7291f90f31b8719348ae4a6b470fbe8d079d0956
Signed-off-by: Lior Vernia <lvernia@redhat.com>