cli: parse CEPH_ARGS with rados.conf_parse_env

It is incorrect to append the content of CEPH_ARGS to the argument list
because it does not take -- into account, pass options to injectargs
instead of using them as expected and probably other undesirable side
effects.

Argument parsing delegated to rados.conf_parse_env() instead.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>