Add analysis_executable property

Currently I get the error below when trying to run pymultinest with mpi through bilby_pipe:

Traceback (most recent call last):                                                                                                                                                          [11/228]
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38/bin/bilby_pipe", line 10, in <module>
    sys.exit(main())
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38-20220524/lib/python3.8/site-packages/bilby_pipe/main.py", line 435, in main
    generate_dag(inputs)
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38-20220524/lib/python3.8/site-packages/bilby_pipe/job_creation/bilby_pipe_dag_creator.py", line 79, in generate_dag
    analysis_node = AnalysisNode(
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38-20220524/lib/python3.8/site-packages/bilby_pipe/job_creation/nodes/analysis_node.py", line 29, in __init__
    parallel_program=self._get_executable_path(
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38-20220524/lib/python3.8/site-packages/bilby_pipe/job_creation/node.py", line 52, in _get_executable_path
    exe = shutil.which(exe_name)
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38-20220524/lib/python3.8/shutil.py", line 1379, in which
    if os.path.dirname(cmd):
  File "/cvmfs/oasis.opensciencegrid.org/ligo/sw/conda/envs/igwn-py38-20220524/lib/python3.8/posixpath.py", line 152, in dirname
    p = os.fspath(p)
TypeError: expected str, bytes or os.PathLike object, not NoneType

This functionality seems to have been broken when the option to pass a generic analysis executable was added. This MR fixes the bug and reinstates the mpi+pymultinest functionality. Thanks to @colm.talbot for the debugging help.