Ensure that we treat the locks in the same order with the
delete and create operations.
The fix above result in a deadlcok due to a change in neutron
where ipam delete call port_update. This conflicted with
commit d89eba1a851d76179bd882cc8024b86f71e1153b.
To ensure locking for this we make use of a new lock for the
DHCP interfaces.
Change-Id: I6c3f25ab40247853024560c00d3faa106e5d90b8