Fixes cinder failed to create/restore a backup with NFS driver

The action cinder backup-create fails when trying to backup a newly
created volume when using cinder.volume.drivers.nfs.NfsDriver. This
patch removes not implemented stubs for backup_volume and
restore_volume. The inherited methods in cinder/volume/driver.py
succeed in creating a backup and in restoring it. After manual
testing of this change backups/restores succeed without the not
implemented stubs. This change also adds the not implemented
stubs to the glusterfs driver until a fix is submitted.

Closes-Bug: #1247743

Change-Id: I5478d00442ce855c91a7f04c5ba2b40733d44902
Merge "Fixes cinder failed to create/restore a backup with NFS driver"