Allow ::/0 for IPv6 security group rules

Nova fails to create a security group rule if ::/0 is used as
value for the cidr field

Fixes: LP bug #1199943
Cherry picked from https://review.openstack.org/36556

Change-Id: Ifce53dbc9f0740337921491aa752900892f47052
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>
Merge "Allow ::/0 for IPv6 security group rules" into stable/grizzly