Leverage neutronclient.openstack.common.importutils import_class

This patch allows us to drop our version of import_class in favor
of openstack.common.importutils version's.

Change-Id: Ia96f06e2dc7dfb9824377e3a7381da2732dd2c15
Closes-bug: 1365273
Merge "Leverage neutronclient.openstack.common.importutils import_class"