To import outputs from stacks that are not managed directly by a Stacker
config and require custom settings (such as a profile or region), it was
previously possible to just omit both a `class_path` and
`template_path`.

Make that pattern "official" by introducing an `external` option, and a
simplified `ExternalStack` model that omits all the stack keys that do
not make sense if we're never going to create/update the stack.

Note: the upgrade to schematics 2.1.0 is required due to a bug [1] where
polymorphic models were not properly validated.

[1]: schematics/schematics#440

Don't ever attempt to update them, but always submit them so outputs can
be grabbed.

This makes handling the difference between managed and external stacks
much easier.

Also clean up the build tests a little bit by creating a TestStack class
instead of using Mocks.