SSP-1001 Accept system properties when executed from Gradle

Set them as a 'sysProps' build property. Don't use the -D syntax.
E.g.:

  %> gradle -q run -PsysProps="user.timezone=GMT" \
     -PcliArgs="--db-url=<db-url>"

As noted in README.md, parsing is naive, so don't expect anything
with spaces to work.