webadmin: Default host for RunOnce dialog enabled

New panel with default host selection is added to the RunOnce dialog.
The appearance is similar to the VM edit dialog Host tab.
The RunOnce dialog host panel is invisible in the User Portal application.

Change-Id: I1906caf40c91e12a56523ddda794c72690947d4c
Bug-Url: https://bugzilla.redhat.com/560981
Signed-off-by: Libor Spevak <lspevak@redhat.com>