Enable mellon diagnostics when debug=True as of Cosmic

/etc/apache2/mods-enabled/auth_mellon.conf needs to include "MellonDiagnosticsEnable On" and "MellonDiagnosticsFile <location>" in order to ease up SSO debugging at the SP side when debug is set to True.

https://bugs.launchpad.net/ubuntu/+source/libapache2-mod-auth-mellon/+bug/1820279
https://bugs.launchpad.net/ubuntu/+source/libapache2-mod-auth-mellon/+bug/1820279/comments/13

# grep MellonDiag /etc/apache2/mods-enabled/auth_mellon.conf
MellonDiagnosticsFile /var/log/apache2/mellon_diagnostics.log
MellonDiagnosticsEnable On

sudo tail -f /var/log/apache2/mellon_diagnostics.log

---------------------------------- New Request ---------------------------------
GET - /v3/OS-FEDERATION/identity_providers/samltestid/protocols/saml2/auth
log_id: (null)
# ...