* Support `distributionPolicy` when creating regional instance group managers.
* Better match the API structure of distributionPolicy.
* Switch to "distribution_policy_zones".
This approach lets us more simply allow a list of zones to use, while
providing a deprecation path for implementing the distribution policy
field more holistically, avoiding backwards-incompatible changes.
* fix typo
* use slice instead of Set for flattenDP
* Add new data source: compute region instance group manager's groups.
* Add documentation for wait_for_instances and for the timeout mechanism in resourceComputeRegionInstanceGroupManagerCreate.