saml_provider configuration doesn't work for multiple servernames

After setting `saml_provider` Apache's mod_auth_mellon is configured and a file is created in /etc/apache2/mellon/{{servername}}.xml. This configuration uses the servername as a return URL for the SAML provider. In case where the vhost has multiple servernames SAML auth can't be used without manual creation of those XML files (for each servername).

For this to work, maybe we could have a new config (extra_servernames) that would be used to create additional mellon XML files (and even other configs elsewhere in the charm) that could be used by the vhosts configured by the user.