DE734: Instance not getting DHCP due to missing of vlan tag on openvswitch port

- Initial refactoring of a patch authored by AT&T
- Moved get_port_tag_dict to ovs_lib.py which seemed more appropriate.
- Now only examining a port to look for a tag on specific types of tenant networks.
- Added tenant_network_type to the initialization of OVSNeutronAgent
- Now checking for specific tenant network types before looking for vlan tags.
- Now setting self.tenant_network_type from the OVS config.
- Adjusted unit test.
- Adjusted log message when port in forced to resync.
- Clean up test values.
- Removed incorrect spacing.

Closes-rally-bug: DE734
Not-in-upstream: true