Allow configuring the "Delete" timeout

With the release of 19.4, group delete goes through a pending process prior to starting deletion (which we saw in the nightly tests), causing it to sometimes take longer to actually fully delete. While this is likely not an issue in normal, production-scale operation, it causes flakiness in our acceptance tests.

This MR updates the Delete operation to be configurable, sets the default to 10 minutes (the current hard-coded setting), and then updates the main flakey test to allow up to 30m for delete instead of 10.