Provide a Keycloak configuration endpoint

To decouple UPS from Keycloak in separated servers a configuration interface must be built to generate the appropriate JSON file on the fly, during the configuration.

Some rules must be respected:

• First time configuration

If the configuration is the first time, just prompt user with the configuration interface built buy agalante_jira in AGPUSH-1110.

• After the URL configuration

If the configuration was already provided and the admin must change it, he must be logged in to change. Otherwise, change to other Keycloak server won't be allowed