Workaround Neutron DHCP scheduling failure.

For some reason the native Neutron PXE setup is failing to schedule
the DHCP agent to handle the ctlplane network. This fugly hack will
address it until we investigate the root cause.

Change-Id: Ieb4b6542ae9298ef395cf3afbf4161d50b7ab62c
Related-bug: #1231366