Main.java shouldn't have hard-coded lists of commands for usage

ACCUMULO-4490 added hard-coded lists of commands in the Main class, for the purposes of usage. However, this regresses what ACCUMULO-3514 was trying to fix.

In addition, the current implementation results in NullPointerExceptions when one of the hard-coded keywords are not found in the classloader.

Main should be fixed so that it does not depend on hard-coded lists of KeywordExecutables.

mikewalch mentioned to me that part of the reason this was done was to affix the ordering of the usage. I think that goal can be achieved by extending the KeywordExecutable interface to include a grouping or ordering attribute instead.