Validate CIDR given as ip-prefix in security-group-rule-create

There was no validation for the provided ip prefix. This just adds
a simple parse using netaddr and explodes with appropriate message.
Also makes sure ip prefix _is_ cidr (192.168.1.1-->192.168.1.1/32).

Validation occurs at the attribute level (API model) as well as at
the db level, where the ethertype is validated against the ip_prefix
address type.

Unit test cases added - bad prefix, unmasked prefix and incorrect
ethertype. Also adds attribute test cases for the added
convert_ip_prefix_to_cidr method

Closes-Bug: 1255338

Conflicts:
	neutron/tests/unit/test_security_groups_rpc.py
	neutron/tests/unit/test_extension_security_group.py

Change-Id: I71fb8c887963a122a5bd8cfdda800026c1cd3954
(cherry picked from commit 65aa92b0348b7ab8413f359b00825610cdf66607)