Create namespaced config for plugins

[mberkowski]

Plugins (like auth/shibboleth) may have their own config, and their own required config options. We could use an API to specify what's required, and to load plugin-specific config into sub-arrays like

$CONF['PLUGIN']['AUTH']['SHIBBOLETH']['ENTITY_IDENTIFIER']

[cpockett]

I'd like that better than adding them to the main config agreed.