[Autoscaler v2] Autoscaler supports virtual clusters [1/N]

[Chong-Li]

Why are these changes needed?

This is the first PR to make autoscaler v2 support virtual clusters. With this PR, one single un-divisible virtual cluster can autoscale by pending tasks.

Some follow-ups:

1. Cover the scenario with multiple virtual clusters.
2. Configure min/max workers for each virtual cluster. We may also need min/max unassigned workers in the primary cluster.
3. Support autoscaling job cluster and divisible virtual cluster.

Related issue number

#506

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• 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 added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• 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 :(