Background of the modification

This patch adds a bunch of connection arguments that are compatible with MySQL Connector/Python.
https://dev.mysql.com/doc/connector-python/en/connector-python-connectargs.html

The rationale for the change is that SQLAlchemy propagates the parameters specified in the DSN's query to the constructor arguments as they are, which means it cannot build a structured parameter to give it to the underlying connection factory.
MySQL Connector/Python can take unstructued TLS parameters so it plays well with SQLAlchemy.

Details

This patch adds the following arguments:

• ssl_ca: Path to the file that contains a PEM-formatted CA certificate
• ssl_cert: Path to the file that contains a PEM-formatted client certificate
• ssl_disabled: A boolean value that disables usage of TLS
• ssl_key: Path to the file that contains a PEM-formatted private key for the client certificate
• ssl_verify_cert: Set to true to check the validity of server certificates
• ssl_verify_identity: Set to true to check the server's identity

This patch also introduces the following key for the dictionary that is supposed to be passed through ssl argument, which effectively closes #842.

• verify_mode: none for ssl.CERT_NONE, optional for ssl.CERT_OPTIONAL and required for ssl.CERT_REQUIRED