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.
can this role be moved into a more generic nfs_client role? Glance may also benefit from the role in the near future as we provide an automated deployment of NFS + netapp for glance. While there is nothing prohibiting glance from using this role in it's present state I think it may be better to name it more generically.