data_refs can describe bind mounts during local docker run

[DXist]

While experiment code is being developed and debugged locally using docker run it might be handy to bind mount local directories with data.

For local docker run I created following override file polyaxonfile.local.yml:

environment:
  data_refs:
    - "~/image_data:image_data_ref"
    - inference_output_data_ref

The changes in this PR propagate data_ref items to POLYAXON_RUN_DATA_PATHS variable and configure bind mounts for data_ref items formatted as HOST_PATH:DATA_REF_NAME. Other data_ref items can be referred in the experiment code by name.

Additionally polyaxon/sdks#30 allows to do local docker runs without Polyaxon server if POLYAXON_NO_OP env variable is set.

[mmourafiq]

This looks good. I think the logic here is growing, and at some point we need to start adding testing.

[DXist]

@mouradmourafiq I committed the change you've suggested.