pybind/mgr/restful: do not start if no certificate is configured

pybind/mgr/restful: do not start if no certificate is configured

This removes the default filename, by the way.  We also work around a
problem with make_server where it sets up the socket to listen before
checking for the cert, thereby making it problematic to rebind to the
port shortly thereafter when we do have a socket.  (SO_REUSEADDR would
be appropriate but there doesn't seem to be an easy way to make
make_server use it.)

Signed-off-by: Sage Weil <sage@redhat.com>