neutron-lib contains a number of the plugin related constants from
neutron.plugins.common.constants. This patch consumes those constants
from neutron-lib and removes them from neutron. In addition the notion
of the dummy plugin service type is moved strictly into the test
package of neutron since it's not a real service plugin.
NeutronLibImpact
Change-Id: I767c626f3fe6159ab3abd6a7ae3cb9893b79bf66
95f1e03446