Rename `Container` templates to `Wrapper`.

Container will be overloaded soon as it cannot be used for both the
widget container used by input() and the wrapping elements for
individual input/label elements.

Refs #3651
Rename groupContainer to inputContainer

I think this will result in nicer conventions once the per input type
containers are added. We'll have templates like `inputContainer` and
`radioContainer` instead of `groupContainer`.

Refs #3651
Add conditional container templates.

Having conditionally used container templates lets developers define
different container HTML for each input type.

Refs #3651
Add tests for datetime container type.
Merge pull request #3702 from cakephp/3.0-form-container-template

3.0 form container template