host: Cleanup containers concurrently

Signed-off-by: Lewis Marshall <lewis@lmars.net>
script: Give the host more time to cleanup jobs

Signed-off-by: Lewis Marshall <lewis@lmars.net>
script: Use unique log file when bootstrapping

Signed-off-by: Lewis Marshall <lewis@lmars.net>
script: Symlink /tmp/flynn-host.log to latest log

Signed-off-by: Lewis Marshall <lewis@lmars.net>