tests: deletes all users between two tests
tests: adds test for user_list
fixes bugs in service_get and service_get_tree

On error VaultError is raised. However it's not imported
anywhere.
tests: Adds test for invalid service get
Fixes tests for user_list and service_get
Adds test for service_put
tests: improves test_group_put
tests: adds tests for machine and customer put and get