Generalized way of saving teuthology suite data to a MySQL database.
Currently, this implements a table for saving tethology suite results,
and a table for saving rados benchmark testing results.

Fixes: 4990
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Removed db_classes.  Took the methods implemented there and made them
functions in results_db.py.  Replaced class references with lists of
tuples.  Fixed docstrings and changed fields to be more Sphinx compliant.
Implemented suggestions made in the previous review.
Changed function name from _config_db to  _connect_db
Make sure we to check that a table exists before trying to access it.  This fixes
a problem which occurs when a table is not available.