Add --worker-port-list option to ray start

[edoakes]

Why are these changes needed?

Allows users to specify a list of specific ports to use for workers. Takes precedence over min-worker-port and max-worker-port.

Related issue number

Closes #11203

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[edoakes]

@richardliaw I did manual testing and added a very simple test to test_multi_node, but couldn't think of a straightforward way to do more complete testing on travis. LMK if you have any ideas.

[richardliaw]

Looks good, just small nit about exceptions

[richardliaw]

In [3]:                 try:
   ...:                     port = int(port_str)
   ...:                 except ValueError as e:
   ...:                     raise ValueError(
   ...:                         "worker_port_list must be a comma-separated " +
   ...:                         "list of integers: {}".format(e))
   ...:
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-160cfabed35e> in <module>
      1 try:
----> 2     port = int(port_str)
      3 except ValueError as e:

ValueError: invalid literal for int() with base 10: 'hi'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-3-160cfabed35e> in <module>
      4     raise ValueError(
      5         "worker_port_list must be a comma-separated " +
----> 6         "list of integers: {}".format(e))
      7

ValueError: worker_port_list must be a comma-separated list of integers: invalid literal for int() with base 10: 'hi'

[richardliaw]

To avoid unnecessary exception chaining, maybe do raise ValueError(...) from None

[edoakes]

wow, I had no idea you could do this! very cool

[richardliaw]

fyi the tests are all failing for some reason