Address cloud provisioning review nits (#6230)

* Use errors.New for the scoped push cred error

The both-or-neither error string has no format arguments, so errors.New
is more appropriate than fmt.Errorf. Addresses a review nit on #6170.

* Update cloud poll logging to reduce noise

WaitForTestRunReady logged the poll interval at Info when the default
was used, and logged every status transition at Info. Neither matches
the rest of the cloud subsystem: no output logs its poll interval (they
are silent constants), "using default X" is only ever a Warn about bad
input, and routine status is shown via a progress bar or logged at
Debug (final status) / Trace (periodic flushes).

Remove the poll-interval log and drop status-transition logging from
Info to Debug. Addresses a review nit on #6169.