webadmin: Limit selection of datacenters in new quota

when creating new quota from the datacenter_quota subtab only the
selected datacenter should be an option for the created quota

and fixed a possible null pionter exception in a certain race conditions

Change-Id: I81c6d13e6ddb704c85593d8ffec317e40a3d3a86
Bug-Url: https://bugzilla.redhat.com/911241
Signed-off-by: Noam Slomianko <nslomian@redhat.com>