boot an instance will fail if use "--hint group=XXXXX"

[David Xie]

When use command "nova boot --image <image name> --flavor <flavor id> --hint group=<group uuid> vm1", booting will fail.
After debugging, I found that there's a code error in nova/scheduler/filter_scheduler.py#174

173 values = request_spec['instance_properties']['system_metadata']
174 values.update({'group': group})
175 values = {'system_metadata': values}

values is not dict, can not use `update` method.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/42377