use service type constants from neutron_lib plugins

The well known service type constants are in
neutron_lib.plugins.constants, but for legacy reasons a few still exist
and are referenced from neutron_lib.constants that we'd like to remove.
This patch switches references over to neutron_lib's plugin constants.

Change-Id: I1861448cec303725b30cef8f42029f467f9e03a3
1 file changed
tree: 098e2c54ea3a3110e36426f02c4485af1a732a19
  1. neutron/