GEOMESA-432 Improved GeoMesa Tools script

This PR addresses some usability issues with the geomesa shell script. Now environment variables
for hadoop and accumulo can be optionally set, and individual paths can be overridden by the user.
Also more informative messages are printed when certain variables are not set.

GEOMESA_HOME and GEOMESA_LIB are now set in function calls, "configure" still works and now should
print out what needs to be added to persist the configuration.