Ensure create_engine() retries the initial connection test

Before return create_engine() tests the connectivity to a DB. At this
moment a DB might be unavailable for some reason, in which case
create_engine() retries a few times. When used with MySQL, we didn't
actually retry, but instead failed on the first connectivity error
when trying to get the value of sql_mode session variable.

Closes-Bug: #1376211

Co-Authored-By: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I14e25cfe1ed51b0d51a94e491b7267f26e42d34e