Enable passing the certificate authority on the relation in order to
enable TLS communication to the MySQL DB.
In order to enable MySQL to use TLS for client connections all that
is required is the CA certificate. A certificate and key may be used
for client certificate authentication. However, since almost all
deployments will not use certificate authentication, focusing on the CA as SSL
data complete makes sense.
Change-Id: I785afe7f64cb57caa857178d529e3cabdcf63517