Make DHCP provider pluggable

Previously, all dhcp was done explicitly through Neutron.  This commit
adds an interface for dhcp providers, currently implemented by a
Neutron provider and a noop provider.

The provider can be set in the dhcp_provider config option, and
defaults to neutron.

Co-Authored-By: Jim Rollenhagen <jim@jimrollenhagen.com>
Co-Authored-By: Lucas Alvares Gomes <lucasagomes@gmail.com>
Implements: blueprint support-external-dhcp
Closes-Bug: #1282168
Change-Id: I7eea490c573914c6cd2a73056fe3a0ec91065dc0
Merge "Make DHCP provider pluggable"