Handle default values for environment variables
 - In crawl.cfg.sample, provide a default value in case $CRAWL is not set
 - Update util.expand() to be able to handle values like ${CRAWL:-.}. If the
   environment variable is unset or empty, this version of expand will replace
   it with the empty string.
 - Add tests for expanding $FOO, ${FOO}, ${FOO:-default} with $FOO unset, set
   to '', or set to 'SOMETHING'
 - btw: Rename test_Util.py to test_util.py
 - btw: Make TAGS depend on hpssic/*.py and hpssic/test/*.py

Change-Id: I8efaf56afa23d78f0acf16bffdb9ac00669a016b