Clustering: Take care of comments in robot test reviews

Two minor cleanups are needed based on review comments that came in after two fixes were merged.

1. Based on comments at:
https://git.opendaylight.org/gerrit/#/c/24255/2/test/csit/suites/controller/Clustering_Datastore/010_crud_on_leader.robot
Remove the following line in 010_crud_on_leader.robot
../../../libraries/RequestsLibrary.py

2. Based on comments at:
https://git.opendaylight.org/gerrit/#/c/24400/2/test/csit/suites/controller/Clustering_Datastore/140_recovery_restart_follower.robot
In the test case "Attempt to add a car to the leader", add 503 as a possible error code along with 500.