MQTT, x.509 certificate-based authentication: use the correct key name for the TLS SAN type configuration parameter

[MarcialRosales]

Proposed Changes

It fixes an issue caused when the mqtt.ssl_cert_client_id_san_type was renamed in the schema to 'mqtt.ssl_cert_login_san_type` to keep it consistent with how the username is determined. But one part of the code was not updated accordingly.

Adding a selenium end-2-end test so that this kind of issues are spotted earlier.

References #12595.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue Cannot set mqtt.ssl_cert_client_id_san_type and san_index #12595)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI