webadmin: Remove coupling between QoS dialog and DC/QoS subtab

So far it was assumed that the dialog is always opened from the DC/QoS
subtab, this coupling is now removed.

Change-Id: I1d9e5f8b5a0c2b197b81ba31e61f28ebc3eeffb3
Signed-off-by: Lior Vernia <lvernia@redhat.com>