Makes following changes to fix and tidy up p2p-versionbits-warning.py:
- add node alias in the run() method
- call versionbits_in_alert_file() in a wait_until loop.
- don't clear out the alert.txt file
- explicitly comment why the node needs to be stop-started
- Verify that the node is out of IBD after stop-start (nodes in IBD do
not generate alert messages)
- no need to subclass P2PInterface
