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 it by properly initializing config_path at all times
We also verify it by adding a variation of an exsiting test that tests the behavior in memecache presence