17014: controller handles container requests

17014: controller handles container requests

This commit has:
  * Container Requests in the new codepath
  * runtime_constraints and scheduling parameters have now default
    values and cannot be nil. This affects Railsapi CR comparison
    in Final state
  * Adapted Login and Logout so they work for CR
  * Tests in  lib/controller/integration_test.go that take in account
    runtime_token with intermediate clusters
  * Some tests in  lib/controller/federation_test.go had to be addapted,
    but for the most part remained as is to gurantee compatibilty
  * Added the check that SystemRootToken has to be non-empty
  * Railsapi fixtures that include finalized CR that are used in the tests to
    make sure we return the right object
  * Minimal changes in the documentation to reflect all the user visible
    changes
  * Added expires_at parameter for login and related test

Arvados-DCO-1.1-Signed-off-by: Nico Cesar <nico@curii.com>