mysql: preserve cleartext password auth setting

[huynhtrungcip]

Changes

• Preserve the MySQL driver default AllowCleartextPasswords setting for generic mysql:// URLs.
• Continue allowing cleartext password authentication in awsmysql, where the connector configures TLS before connecting.
• Add coverage for default, explicit false, and explicit true allowCleartextPasswords URL settings.

Tests

• go test ./mysql -run TestConfigFromURL -count=1
• go test ./mysql ./mysql/awsmysql ./mysql/azuremysql -count=1
• go test ./mysql/... -count=1