Add waiter models

[jamesls]

This re-adds #985, and fixes a bug where creating a client without a region would sometimes generate an error. The fix here is so use the waiter_model (via the newly added get_waiter_model from botocore). From the newly added commit:

Update waiters to use the new get_waiter_model

This fixes an issue where previously we were creating a client
without specifying a region, which in certain cases would trigger
an error.  Now we only use the waiter model to generate the necessary
commands.

As part of thie change I went ahead and just switched the invoke
to just use the service/operation object.  This isn't ideal, but at the
same time, it's pretty clear that for the time being there's no way
to do this entirely with just clients.  Given that's the case, I don't
feel it's that imperative to use clients where possible.  When
we switch to clients, this module will need updates regardless.
It also simplified the code a little bit.

Also, the CI build will fail until the get_waiter_model method is merged,
(depends on boto/botocore#376).

cc @kyleknap @danielgtaylor

[kyleknap]

Looks good. 🚢 Just had the small pep8 comment. I agree with your point on waiting on using the client interface for waiters. I feel that we need to make the conversion first in the clidriver before trying to use clients in the customizations. The customizations revolve too much on the clidriver to be able to make the transition in the customizations first.

Also, I do not think there is a changelog entry for this. We should add one.