Add method for deallocating networks on reschedule

The original call to dhcp_options_for_instance() is not a great way to
check whether deallocation of networks should happen on reschedules.

This adds a new virt driver method 'deallocate_networks_on_reschedule'
which can be used by a virt driver to say whether reschedules should
deallocate networks first. This defaults to False and modifies the
baremetal virt driver to return True. Ironic virt driver will also need
to return True.

Closes-Bug: 1342919
Change-Id: I54a3252ab15e2d8b596ccc90eb4755405021f1da
Merge "Add method for deallocating networks on reschedule"