Ansible: Update documentation, Code Cleanup (and Extend Log Verbosity option)

First commit (8a925e3) absolutely matters since users will very soon start playing with Vagrant 1.3.x and Ansible 1.3.x !!! I think that now all options are covered in the documentation, but please double check my english is understandable!

Second commit (ce4f282) is matter of taste, and I apologize to make so much noise around such little thing... (feel free to request for change or even reject this part). I would thus argue following about log verbosity (sorry to be so *verbose* :ant: :ant: :ant: :ant: :ant: on this topic ;-) :
 * it absolutely makes sense to support all three levels (`-v`, `-vv` and `-vvv`)
 * At the moment, I'm pretty sure that there is no way to mute `ansible-playbook`. I mean that `-v` (or `--verbose`) is the minimal verbosity level, even if Vagrant `ansible.verbosity` option is not defined.
 * I find easy to tweak verbosity with `v+` style, since it corresponds to `ansible-playbook` arguments. Parsing could be even more tolerant and also accepts `ansible.verbose = '-v'` (optional dash)... as you want.
 * I don't like very much the alias logic (`extra`) since there is no obvious mnemotechnic, but as long as we support `v+` style it does not hurt. On the other hand, I agree that `v+` style apply more to  (old-style) CLI world, and that providing "words" to human beings is fair :monkey_face:
 * If we keep human readable aliases, I would vote for an alias map, looking like following:
   * `v` = `normal` (useless as commented above)
   * `vv` = `high` (or `extra`, or `intermediate`, or what ever)
   * `vvv` = `maximum` (`maximal`, or `max`, or `highest`, or what ever)
 * last but not least, It could be elegant to unify the log level names across the different provisioners supported by Vagrant (but it looks like a long way to go after shortly looking at how Shell, Chef, Puppet, etc. are currently implemented)

/cc to people who commented #1979, feel free to shoot your opinion: @bmabey, @commandtab, @jarv.