Refactoring neutron ml2 plugin l2pop driver config opts to be in
neutron/conf/plugins/ml2/drivers/l2pop so that all the
configuration options for l2pop drivers reside in a centralized
location. This simplifies the process of looking up the l2pop driver
config opts and provides an easy way to import.
Change-Id: Ia06c0c31ef40c691d67cfc5d514fdff907f5dca4
Partial-Bug: #1563069