Fix RestController 500 errors

This problem exists in both the RestController and the ApiController.

*Controller.load_config was returning too early if the
`cluster_config` was retrieved from memcached. Later in the
controller call chain, if `cluster_config` is set, `config_path` is
expected to be set as well, but the `load_config` call was returning
prematurely before `config_path` could be set.

This was resulting in intermittent generic 500 errors when exercising
the /api/ URL REST features. (Basically, a reqest would work the first
time, then throw errors until the cache expires--about 60 seconds
later--and then start working again.)

This patch fixes that and adds tests to make sure the proper instance
attributes are being set.