I'm not really sure why I originally had --logfile also log to stdout
in I202e1cb200bde17f6d7770cf1e2710bbf4cca64c, but it seem
counter-intuitive (indeed, I just tripped myself up thinking that in a
devstack job "--logfile" would put the logs into a separate file and
avoid the stdout logging, and I wrote it!).
Make it so specifying a --logfile puts dib into quiet mode for stdout.
Explicitly overriding DIB_QUIET will allow both if someone wants that.
Change-Id: I3279c9253eee1c9db69c958b87a0ce73efc0be9b
