Token caching with custom profiles

Token caching is implemented in `faculty.session.get_session`. But this calls `faculty.config.resolve_profile` which has a pre-defined order of places it pulls credentials from. It would be better if `faculty.session.get_session` takes in a `faculty.config.Profile` instance (or caching is implemented without having to always force resolving profiles). For example, on a platform server that has environmental variables defined, passing `credentials_path` to `faculty.session.get_session` has no effect.

Note that for my example there is a somewhat ugly workaround:

```
import faculty
import faculty.session
import faculty.clients

profile = faculty.config.load_profile('/path/to/config', 'default')
session = faculty.session.get_session(
domain=profile.domain, client_id=profile.client_id,
client_secret=profile.client_secret
)
client_class = faculty.clients.for_resource('cluster')
cluster = client_class(session)
```