More jenkins recon
 - README.md: documenting environment variables that get used
 - Look for the plugin dir in $CRAWL (the root)
 - Put incidental outputs (default log file, etc) in $CRAWL/work
 - Does ${CWD} look as we expect just before running the tests?

Change-Id: I26d6a58a70fcf532b6adf4b633a8380e731ed0c8