This change updates various agent cleaning functions to also
support deploy steps via a new step_type argument. It does not
yet make it possible to run in-band deploy steps.
The agent's get_clean_steps call has been modified to not fail
if there are no cached steps. This is going to be normal for
both cleaning and deploy in the future, and it is impossible
to hit now because of the way cleaning is started.
Change-Id: I789b130e7e490e23924338a024397973957272ac
Story: #2006963
16bce7f18d