Better support for OIDC without the discovery endpoint

[sberyozkin]

We've had jwksPath and instrospectionPath properties since the very beginning but they've never worked and have only confused the users, due to the fact we have always relied on the auto-discovery

This PR:

• introduces a discovery-enabled property (enabled by default)
• if it is disabled - makes sure the minimum set of OIDC endpoint properties is set for web-app/service and initializes OAuth2Auth correctly
• adds a test (relies on the test OidcResource which has a jwk endpoint)

CC @MarcusBiel