core,webadmin: Show Pools tab based on tree

Tab Pools was shown only for System node when selected
in the left tree panel.

New support for Datacenter and Cluster filtering
is available.

oVirt query language had to be extended in new syntax:

Pools: cluster = XXXX
Pools: datacenter = XXXX

Change-Id: I39c39366865ff6a62d30fab993685c72e0c39e10
Bug-Url: https://bugzilla.redhat.com/846284
Signed-off-by: Libor Spevak <lspevak@redhat.com>