NFS client configuration for cinder (fixes #506)

Adds the NFS client configuration in the cinder_volume hosts, which is necessary when using a netapp backend over NFS.
For this conditional I think it may be better to simply look for the defined cinder_nfs_client variable because a user may have multiple netapp backends and name them differently based on internal company policy.

``` jinja2
{% if cinder_nfs_client is defined %}
```