Skip xml test test_schedule_pool_with_down_agent

This patch causes the above test to be skipped as it has caused the havana
tests on the gate to be broken for a few weeks now. Since XML support doesn't
fully work in neutron and marked to be removed in juno skipping this xml test
should be okay.

Change-Id: I20fdf88ae129f77fa0a0ead9b552b4c4adfebbfc
Related-bug: 1311232
Merge "Skip xml test test_schedule_pool_with_down_agent" into stable/havana