Support Nova scheduling hints in OpenStack CPI

These hints will be passed to Nova through fog's `os_scheduler_hints` parameter.

ex:
```
resource_pools:
  - name: common
    network: default
    size: 1
    stemcell:
      name: bosh-openstack-kvm-ubuntu
      version: latest
    cloud_properties:
      instance_type: m1.small
      availability_zone:
      scheduler_hints:
        group: af09abf2-2283-47d6-f2bd-2932a9ae949c
```
what happens when `resource_pool['scheduler_hints']` is nil? Does fog/openstack allow that value?