Prepare to make get_all_instances actually return all instances

`get_all_instances` doesn't actually get all instances, but rather gets
all *reservations*, which themselves contain instances.  This commit
starts the process of making `get_all_instances` actually return a list of
instances by doing a few things:

1. Rename the current `get_all_instances` to `get_all_reservations`.
2. Make `get_all_instances` a stub that warns and then calls
   `get_all_reservations`.
3. Add a new `get_only_instances` method that will eventually become the
   new behavior of `get_all_instances`.

People can then transition to get_all_reservations and
`get_only_instances` as appropriate until `get_all_instances` finally
switches over in a future release.