Introduce {cont_dspath} to contain what {img_dspath} was and fix {img_dspath}

[yarikoptic]

img_dspath to point to actual ds containing the img. Despite description in the containers-add saying that it points to the dataset containing the image, it is not necessarily the case whenever images are contained in a subdataset of the dataset which carries definitions of the container. So, this PR fixes that and also adds cont_dspath which would then contain relative path to the dataset containing the definition of the container. This way it would be possible to enable both cases - referencing commands within subdataset containing image or containing the definition of the container.

It came up in the context of ReproNim/containers#86 where I am thinking to copy definition of containers into super-dataset so it would make it easy to "freeze" them in superdataset while leaving subdataset with actual container definitions without modifications.

TODOs:

• add formatting of img_dspath etc based on cont_dspath.
• get away from naive get_dataset_root to logic which would ensure first that we have installed all subdatasets leading to that location first since otherwise we simply cannot know the sub dataset actually containing the image.

[qlty-cloud-legacy]

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[yarikoptic]

travis is green but appveyor reported following which seems generic absence of singularity - ie unrelated

=========================================================================================================== short test summary info ============================================================================================================
FAILED ../tests/test_run.py::test_container_files - datalad.support.exceptions.IncompleteResultsError: Command did not complete successfully. 1 failed:
[{'action': 'run',
  'exception': CommandError(''),
  'exception_traceback': '[run.py:_execute_command:691,runner.py:run:223]',
  'exit_code': 127,
  'message': 'singularity exec righthere ls',
  'msg_path': '/home/appveyor/DLTMP/datalad_temp_test_container_files80adhv_p/.git/COMMIT_EDITMSG',
  'path': '/home/appveyor/DLTMP/datalad_temp_test_container_files80adhv_p',
  'run_info': {'chain': [],
               'cmd': 'singularity exec righthere ls',
               'dsid': '41d24fd0-73ab-45a5-a49a-5e39d121abee',
               'exit': 127,
               'extra_inputs': ['righthere'],
               'inputs': [],
               'outputs': [],
               'pwd': '.'},
  'status': 'error',
  'type': 'dataset'}]
FAILED ../tests/test_run.py::test_run_no_explicit_dataset - datalad.support.exceptions.IncompleteResultsError: Command did not complete successfully. 1 failed:
[{'action': 'run',
  'exception': CommandError(''),
  'exception_traceback': '[run.py:_execute_command:691,runner.py:run:223]',
  'exit_code': 127,
  'message': 'singularity exec .datalad/environments/d...',
  'msg_path': '/home/appveyor/DLTMP/datalad_temp_tree_test_run_no_explicit_datasetvjow90c1/.git/COMMIT_EDITMSG',
  'path': '/home/appveyor/DLTMP/datalad_temp_tree_test_run_no_explicit_datasetvjow90c1',
  'run_info': {'chain': [],
               'cmd': 'singularity exec .datalad/environments/deb/image cat '
                      '{inputs[0]} {inputs[0]} >doubled',
               'dsid': 'd6ecd6a4-d716-4f5c-b5f4-1913bc06787d',
               'exit': 127,
               'extra_inputs': ['.datalad/environments/deb/image'],
               'inputs': ['subdir/in'],
               'outputs': ['doubled'],
               'pwd': '.'},
  'status': 'error',
  'type': 'dataset'}]
FAILED ../tests/test_schemes.py::test_docker - FileNotFoundError: [Errno 2] No such file or directory: 'singularity'

appveyor is also red in #200 so - must be unrelated

[yarikoptic]

get_dataset_root is likely not enough here! We must first ensure that there is a subdataset there, so pretty much perform the same trill as datalad get does to install all subdatasets leading to that path. Will need to check for that later and add a dedicated test for that to have hope to get this PR "proper".