Add a testing IDP for OIDC authentication

[enrichman]

The OIDC authentication is missing tests related to groups. The staticPasswords field in the dex config doesn't support groups, and it has only "fixed" users.

Setting up an external IDP (LDAP?) will allow us to create/update users, and make some checks related to their groups and permissions. We would need to update the dex configuration, and restart Dex though.

Needs investigation.

[enrichman]

Pairing with @mmartin24 maybe we don't need a full blown IDP to test some of the OIDC features, such as groups. We can create a valid JWT token with the needed claims to test the groups association.

[mmartin24]

@enrichman, I made this Confluence page with a summary of the steps done with this: https://confluence.suse.com/display/Epinio/How+to+check+update+of+user+roles+on+each+new+login+session