add PROVISIONAL_IPV6_PD_PREFIX to constants

The constant PROVISIONAL_IPV6_PD_PREFIX was missed when rehoming
neutron.common.constants into lib, and is used outside of neutron [1].
This patch adds the constant to lib.

[1] http://codesearch.openstack.org/?q=PROVISIONAL_IPV6_PD_PREFIX

Change-Id: I9642ed9b513a43c5558f9611f43227299707284a

neutron_lib/constants.py

@@ -320,6 +320,8 @@ INGRESS_DIRECTION = 'ingress'
 EGRESS_DIRECTION = 'egress'
 VALID_DIRECTIONS = (INGRESS_DIRECTION, EGRESS_DIRECTION)
 
+PROVISIONAL_IPV6_PD_PREFIX = '::/64'
+
 
 class Sentinel(object):
     """A constant object that does not change even when copied."""

releasenotes/notes/rehome-ipv6pdprefix-const-d3b39992df4adef8.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - The ``PROVISIONAL_IPV6_PD_PREFIX`` constant is now available in
+    ``neutron_lib.constants``.