Add a validation format "cidr"

Nova accepts CIDR attributes on network REST APIs and checks its format
on each API. On v2.1 API, the attributes will be checked with JSONSchema.
In addition, Neutron contains its own "cidr" check for its REST APIs.
This patch add a validation format "cidr" by porting Neutron's check from
https://github.com/openstack/neutron/blob/master/neutron/api/v2/attributes.py#L293

Partially implements blueprint v2-on-v3-api

Change-Id: I03601f8fa34fe7617e4f963cdfd4b178eae36565
Merge "Add a validation format "cidr""