Add --log-level support - Issue #5198

I noticed that even if I set the log level to 'fatal' there are still quite a few things printed to stdout/err.
In my quick scan its because not everything is using the default logger.  For example, I noticed that
job.go has its own.  I think that should be removed and everyone should use the same logger.
I could move that change into this PR if people want, but I'd prefer to split it out into a new one so that
this one is just focused on adding the --log-level flag.

Closes #5198

Signed-off-by: Doug Davis <dug@us.ibm.com>
We're kind of in a weird situation here. Ideally, "lvl" wouldn't be there at all.  I really want to ask logrus what the current level is instead but I don't believe that option is available to us. I've submitted a PR to expose it.

I modified the code slightly - see if you like that better.  There's still one if-statement in there (the nested "if" in the -D processing) that (right now) will always evaluate to "true" but I really want to keep it because if someone modifies the default Docker logLevel and a more new (more verbose than debug level is created) I didn't want people to have to "remember" to check the -D processing and add the if-stmt.  There's no harm in keeping it and the compiler will optimize it out anyway, but future-proofs the code.

Besides, eventually I want to remove the -D/--debug flags entirely and force people to --log-level. Having two ways to do the same thing isn't good.