support variable expansion

Hi,

variables:
  OS_APPLICATION_CREDENTIAL_ID: "@url@http://vault-secrets-provider/api/secrets/${CI_PROJECT_ID}-${CI_PROJECT_NAME}/openstack?field=OS_APPLICATION_CREDENTIAL_ID"

something like this does not seem to work. If I replace CI_PROJECT_IDand CI_PROJECT_NAMEwith their values it works.

Any ideas?