Refactor update_port in db_base_plugin_v2

Refactor update_port in db_base_plugin_v2

This commit is a preparation step for using pluggable IPAM.
- moved validations into _validate_port_for_update;
- updating ip addresses for port is backend specific, so
  moved into _update_port_with_ips in ipam_non_pluggable_backend;
- writing port changes to db is common for both backends, so
  moved into _update_db_port in ipam_backend_mixin;
- updated to use namedtuple to track add/original/remove ips;
- added _make_fixed_ip_dict to exclude keys other than
  ip_address and subnet_id;

Partially-Implements: blueprint neutron-ipam

Change-Id: I1110e88f372b1d0cc7ec72049ba69a6d548da867