(QENG-1211) Added documentation & tests for retry_on

Before, the retry_command method was private by convention, because it didn't have
tests or any documentation.  Some of the QA team wanted to use this functionality,
so now we've added those supporting items to gain confidence in its regular use.
(QENG-1211) Fixed logical error in acceptable exit code argument for retry_on
(QENG-1211) Removing unnecessary description parameter from retry_on

Before, the description parameter was unused by retry_command, but there were usages of the
function that tried putting information into this. Removing it makes the usages more correct,
since it wasn't actually being used, and makes it conform better to our _on function's
parameter standard
(QENG-1211) fixed whitespace issues
Merge pull request #502 from kevpl/qeng1211_retry

(QENG-1211) Added documentation & tests for retry_on