GEOMESA-432 Improvements to GeoMesa Tools script 

GEOMESA-432 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. 
It would be nice if we could just default to the dir the script is run from to find GEOMESA_HOME if it's not set. That is SOP for command line scripts.