[CLI] Support --concurrency without value, defaulting to all instances.

This commit adds a preference for the `--concurrency` option over the
`--parallel` option. A note is added to the CLI help warning that
`--parallel` will be deprecated in a future release.

The following formats are now supported:

    kitchen test -c               # defaults to instances.size
    kitchen test -c=2             # sets max concurrency to 2
    kitchen test --concurrency    # defaults to instances.size
    kitchen test --concurrency=3  # set max concurrency to 3
    kitchen test --no-concurrency # same as the default, serial mode

References #293
References #277