Clusterbuster: allow all objects to be created with a prefix

[RobertKrawitz]

Clusterbuster currently uses the namespace or the workload name (as appropriate) as the prefix to all objects. Allowing all objects to be named with a unique prefix would allow distinguishing jobs that all use the same namespace. For example:

--object-prefix=mytest

would result objects named

clusterbuster-0/mytest-...

Combined with a single namespace for all objects, a single Prometheus snapshot for an entire run, running each job within a run with a distinct prefix would simplify determining where within a snapshot to inspect would be easier.

[RobertKrawitz]

Fixed by #24