webadmin,userportal: Show data centers and their clusters based on configured cluster service

The New VM/Edit VM/Import VM/New Template/Pools dialogs should provide:
- just DC with at least one cluster with enabled Virt Service option
- and only clusters with enabled Virt Service option

Create Volume dialog:
- just DC with at least one cluster with enabled Gluster Service option
- and only clusters with enabled Gluster Service option

Change-Id: Ifb63c7e2addf525bb527d94694a8dfc752d69154
Bug-Url: https://bugzilla.redhat.com/882928
Signed-off-by: Libor Spevak <lspevak@redhat.com>