Move nova-manage.rst to cinder-manage.rst.

Move nova-manage.rst to cinder-manage.rst.

Cinder docs were not building because sphinx was looking for
cinder-manage.rst which did not exist. The file was simply misnamed
and renaming it to cinder-manage.rst allows the docs to build.

Change-Id: I68f4ed81230cc277a96bb34dfa172e5ce7788ac4