Tweaks to scripts/guacamole script for running locally

 * Invoke guacamole in a way that supports java 7 (previously it only worked in java 8).
 * Remove note in the README saying we only support Java 8.
 * Run Guacamole with a single spark task (previously it ended up setting tasks to the number of processors, which could require too much memory)
 * Give a more helpful error if the script is run from the wrong place or when Guacamole is not yet built.