The ceph 'rbd' library (librados) now supports Python 3 and, by
extension, unicode strings [1]. There is no need to keep these encode
calls around: if a user is running nova under Python 3, then it is safe
to assume that they are using a Python 3-compatible version of librados.
[1] https://github.com/ceph/ceph/blob/v11.1.0/src/pybind/rados/rados.pyx#L30-L36
Change-Id: Ic1afb3e66bd0991047f97fa74f9d2fbbb837401a