Make AWSWorker optional

[rth]

Currently when pip installing ramp-board there is an import of botocore and boto3 (which is not included in the dependencies), that produces,

  File "/home/ramp/ramp-board/ramp-engine/ramp_engine/__init__.py", line 1, in <module>
    from .aws import AWSWorker
  File "/home/ramp/ramp-board/ramp-engine/ramp_engine/aws/__init__.py", line 1, in <module>
    from .worker import AWSWorker  # noqa
  File "/home/ramp/ramp-board/ramp-engine/ramp_engine/aws/worker.py", line 5, in <module>
    from . import api as aws
  File "/home/ramp/ramp-board/ramp-engine/ramp_engine/aws/api.py", line 11, in <module>
    import botocore  # noqa
ModuleNotFoundError: No module named 'botocore'

I think ideally we should make it possible to install ramp-board without using the AWS worker and install the corresponding dependencies particularly that AWS has a tendency to pin all requirements which for a library is not ideal nevermind they seem to have improved that situation.

Rather it should an extra, so it can be installed with,

pip install ramp-engine[aws]

or something similar, same as it is currently done for the dask worker.

[agramfort]

👍