Make quiet mode not mute warnings and errors.

Before -q (quiet mode) would mute not only 'info' type
of messages but also errors and warnings, with
high parallelism and quiet flag it was impossible to
find remote boxes that rex failed to execute to.

If one really desire muting everything, should consider
redirecting stderr and stdout to /dev/null.
updated help message
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merged #294 - added another cli param for this: rex -qi ...