Stub provider standardize 'status' field

[bfcrampton]

Currently the StubJobProvider stores operation statuses as a tuple with the status string and date. This causes consumers of dstat to add provider-specific logic to parse out the status:

job =  dstat.dstat_job_producer(provider=provider, ...).next()[0]
status = job['status'][0] if isinstance(provider, stub.StubJobProvider) else job['status']

The output format from StubJobProvider should match the other providers for consistency.