create group policy validation bugfix

When multiple policies are sent in create group request, only first policy was getting validated but the whole
request was getting stored. When scheduler policy is processed it causes exception and 500 error.

Changed to validate all elements in the array. Please see
http://tools.ietf.org/html/draft-zyp-json-schema-03#section-5.5
tests added for create_group validation
Merge pull request #368 from rackerlabs/create_group_validation

create group policy validation bugfix