I renamed `tlsAuth` to `tlsClientAuth` to better describe the fact that
this variable is used to enable TLS client authentication (as opposed to
server authentication) in c04d95f35.

However, changing the name breaks backwards compatibility for existing
installations using this feature and Grafana does not have a
standardised way of migrating changes in the schema:
https://github.com/grafana/grafana/pull/9377#issuecomment-333063543

For reasons of expediency given the severity of the bug (not verifying
TLS), keep the old name.