Cinder-specified volume options for NFS/GlusterFS

The aim of this blueprint is to enhance the way NFS and GlusterFS volumes are handled. Currently, it is possible to set mount options for both Nova and Cinder on a per-node basis in the service config files. This blueprint extends this idea to be able to handle options for each export to be mounted that are set in the Cinder configuration and passed to Nova at attach time.

This is a useful change for GlusterFS because the backupvolfile-server option enables a mount to succeed even if the first server specified is offline. It also may enable more specific performance tuning or deployment options for NFS/GlusterFS volumes.

The configuration on the Cinder side is done on a per-export basis in the nfs_shares_config or glusterfs_shares_config file, i.e.:

192.168.175.166:/testvol -o backupvolfile-server=192.168.175.177
host1:/testvol2 -o backupvolfile-server=host2,rw,other_option

This information will then be passed to Nova in the "data" field returned from initialize_connection() when an attach is performed.

Nova review: https://review.openstack.org/#/c/29325/
Cinder review: https://review.openstack.org/#/c/29323/