Start moving get_all_instances to get_all_reservations

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.