Use 400 instead of 422 for security_groups v2 API

For any invalid request format we should return 400, not 422,
this patch fixes this case for v2 security_groups API.

This patch follows the rule:
https://wiki.openstack.org/wiki/APIChangeGuidelines

Change-Id: I78f523901918901e55b339d0cbd9cc78dd52044f
Closes-Bug: #1380456
Merge "Use 400 instead of 422 for security_groups v2 API"