Cleanup nodes in state DELETE immediately.

The 900 second delay before deleting them from periodic cleanup just
causes excessive waste when we've restarted nodepool.

To avoid concurrent sqlalchemy access (particularly mutations) to node
objects conditions with other per-node operations, this delegates all
deletions to periodic cleanup.

This is done because with the high latency on API command submission
due to low rate limits with providers (e.g. 2 requests/second) it's
likely that a node deleted during busy periods will not have it's API
request *submitted* for up to 5 minutes. As such, the 5 minute cleanup
thread will actually submit the API requests with little or no
increase in latency. And of course that frequency could be increased,
permitting faster cleanup in low-load situations.

Change-Id: Iad7004b0d3389df1e1b687209fd5f6ed27b03239
Log how long nodes have been in DELETE state.

We have providers that are terrible at deleting.

Change-Id: Ia1e5466b607010e7f21f321775aa25ce9543f046
Merge "Cleanup nodes in state DELETE immediately."
Merge "Log how long nodes have been in DELETE state."