Refactor awkward logic in setup_dhcp_port

Refactor awkward logic in setup_dhcp_port

I noticed this logic as I was reviewing another patch set [1].  I
didn't like removing subnet ids from dhcp_enabled_subnet_ids and I
wasn't too keen on the ips_need_removal semantics that were kind of
forced by the existing structure of the code.  I hope you find this
alternative much clearer.  I like straight-forward code with less
indentation that doesn't use awkward booleans like ips_needs_removal.

[1] https://review.openstack.org/#/c/157697/6

Change-Id: I8bd3d6924a855ea08f8096e66bd3bfbb165a4da3