This gets us a rid of the need to configure the reva jwt secret in the
proxy. Also we no longer need to fake an internal admin user for
autoprovsioning user and/or assigning the roles to users from oidc
claims.
Allow to switch jwt access token verification and off. Many (most?) IDP
provide JWT encoded access tokens. If ocis is configure to assume jwt
access tokens (access_token_verify_method==jwt) we now properly verify
the tokens signature and a set of standard claims ("exp", "iat" and nbf"
by way of the jwt module's standard verification and "iss" explicitliy).
This change also allows for introduction of other access token verification
mechanism in the future (e.g. through introspection (RFC7662).
