Add flag to `kitchen init` to skip Gemfile creation by default.

This adds a `--create-gemfile` flag (and its reverse of
`--no-create-gemfile`) to `kitchen init` with a default false value. As
discussed with @schisamo, this should help in situations where Test
Kitchen has been installed as a gem and not necessarily brought in using
Bundler. The updated behavior is as follows:

* If a Gemfile exists, assume we want to use that and append gems to the
  Gemfile. Display a message instructing the user to `bundle install`.
* If no Gemfile exists, issue `gem install` commands to install the
  drivers. The default value of --create-gemfile is false.
* If --create-gemfile flag is present (value of true), create and append
  to a Gemfile. Display a message instructing the user to
  `bundle install`.