Auth providers should be configurable via properties files outside of plugins

In the initial release of XNAT 1.7, authentication providers were only configurable by adding properties files to plugins. There should be a way to modify them without having to edit a jar. For 1.7.1, we want users to be able to put auth provider properties files in the config directory and have them take effect. If there are any providers configured in the config directory, those should be what XNAT uses. If not, XNAT should use any providers configured via plugins. If none are configured there, XNAT should have only the default database provider.