Reduce the cardinality of scap timers/statsd metrics

cli.py:
Allow callers to pass stats=None to Application.Timer() to disable
reporting of stats to statsd.

deploy.py:
Don't report stats for with self.Timer(display_name), where display_name is computed
from the scap deploy operational flags, repo name, and commit hash.

kubernetes.py:
Don't report stats for with self.app.Timer("Running {} in {}".format(" ".join(cmd), dir)).

prep.py:
Don't report stats for with self.Timer("scap prep {}".format(self.arguments.branch)).

Bug: T377883