doc/conf.py: run ditaa with java

doc/conf.py: run ditaa with java

just in case, otherwise we could have

  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/42577/lib/python3.8/site-packages/sphinxcontrib/ditaa.py", line 200, in html_visit_ditaa
    render_ditaa_html(self, node, node['code'], node['options'])
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/42577/lib/python3.8/site-packages/sphinxcontrib/ditaa.py", line 177, in render_ditaa_html
    fname, outfn = render_ditaa(self, code, options, prefix)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/42577/lib/python3.8/site-packages/sphinxcontrib/ditaa.py", line 141, in render_ditaa
    p = Popen(ditaa_args, stdout=PIPE, stdin=PIPE, stderr=PIPE)
  File "/home/docs/.pyenv/versions/3.8.6/lib/python3.8/subprocess.py", line 854, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/home/docs/.pyenv/versions/3.8.6/lib/python3.8/subprocess.py", line 1702, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
OSError: [Errno 8] Exec format error: '/usr/bin/ditaa'

Signed-off-by: Kefu Chai <kchai@redhat.com>