ACCUMULO-2621 Add delay to Shutdown step of continuous randomwalk, update README

The Concurrent.xml randomwalk module can shut down and restart the Accumulo cluster during
the test. This commit adds a 10 second delay after shutdown, to give the master plenty of
time to exit before the test attempts to start it up again.

Also, it is essential that the walker(s) running the test have passwordless SSH access to
the cluster to run $ACCUMULO_HOME/bin/start-all.sh. Otherwise, the masters will not restart
and the test will eventually get stuck. The randomwalk README.md now has information about
that.
ACCUMULO-2657 Properly interpret default empty string ABSOLUTEPATH config properties

This commit alters how the default value for an ABSOLUTEPATH configuration property is
determined.

* The property no longer needs to be annotated @Interpolated for it to be fully
  processed, i.e., for File.getAbsolutePath() to be run on it.
* Processing does not occur when the property default value is an empty string, so the
  default value remains an empty string instead of being processed into the value of
  System.getProperty("user.dir").