GenericJourney: Use json feeder to auth requests

[dmlemeshko]

At the moment, every scenario includes calling internal/security/login end-point. It has few downsides:

• we depend on security service scalability
• we can't test individual end-points if it requires authorisation
• we can't test individual user flow without doing login

The solution is to pre-generate vaild cookies and use Gatling feeder to pass it to user sessions and skip login