Add config options for cinder nfs backend

Currently the cinder nfs backend leaves out few options in a
multi backend deployment. It works in single nfs backend deployment
as devstack-plugin-nfs correctly configures all options[1].
We can clearly see the difference between what devstack-plugin-nfs
configures[1] and what devstack nfs configures[2].

Following options are missing which are added by this patch.
* nas_host
* nas_share_path
* nas_secure_file_operations
* nas_secure_file_permissions
* nfs_snapshot_support

[1] https://github.com/openstack/devstack-plugin-nfs/blob/dd12367f90fc86d42bfebe8a0ebb694dc0308810/devstack/plugin.sh#L60-L68
[2] https://github.com/openstack/devstack/blob/a52041cd3f067156e478e355f5712a60e12ce649/lib/cinder_backends/nfs#L32-L34

Change-Id: I03cad66abb3c6f2ae1d5cf943ac952a30961f783

lib/cinder_backends/nfs

@@ -32,6 +32,15 @@ function configure_cinder_backend_nfs {
     iniset $CINDER_CONF $be_name volume_backend_name $be_name
     iniset $CINDER_CONF $be_name volume_driver "cinder.volume.drivers.nfs.NfsDriver"
     iniset $CINDER_CONF $be_name nfs_shares_config "$CINDER_CONF_DIR/nfs-shares-$be_name.conf"
+    iniset $CINDER_CONF $be_name nas_host localhost
+    iniset $CINDER_CONF $be_name nas_share_path ${NFS_EXPORT_DIR}
+    iniset $CINDER_CONF $be_name nas_secure_file_operations \
+        ${NFS_SECURE_FILE_OPERATIONS}
+    iniset $CINDER_CONF $be_name nas_secure_file_permissions \
+        ${NFS_SECURE_FILE_PERMISSIONS}
+
+    # NFS snapshot support is currently opt-in only.
+    iniset $CINDER_CONF $be_name nfs_snapshot_support True
 
     echo "$CINDER_NFS_SERVERPATH" | tee "$CINDER_CONF_DIR/nfs-shares-$be_name.conf"
 }