Update connect args for pymysql 1.0.0; aiomysql fixes

Update connect args for pymysql 1.0.0; aiomysql fixes

Fixed deprecation warnings that arose as a result of the release of PyMySQL
1.0, including deprecation warnings for the "db" and "passwd" parameters
now replaced with "database" and "password".

For the 1.4 version of this patch, we are also changing tox.ini
to refer to a local branch of aiomysql that fixes pymysql
compatibility issues.

Fixes: #5821
Change-Id: I93876b52b2d96b52308f22aeb4f244ac5766a82f