Allow to configure authentication mechanism per configuration file
This will allow to deploy on staging with keycloak while continue using the deposit with http basic authentication in production.
If some urgency fix is needed, we won't be forced into migrating keycloak immediately to fix that potential other issue.
Note that the following transitional code will probably go away when we are done migrating to keycloak.
Related to #2858 (closed) Related to swh/infra/puppet/puppet-swh-site!322 (closed)
Test Plan
tox
in docker
Migrated from D5291 (view on Phabricator)