ACCUMULO-1472 Rely solely on classpath from the invoking application and prevent extra warnings from being logged.

Create lib/ext so that the classloader doesn't WARN. Since the applications
will inherit the classpath from the invoking application, we shouldn't try
to pull jars from environment vars (as they will rarely be correct) and add
them to the classpath too. Sadly, we must set *something*, otherwise the
AccumuloClassLoader will revert back to some default and load things
we don't want.