proxyquery: dont directly import cache_from_env

Directly importing functions makes it difficult/impossible to mock
things in tests.
proxyquery: config_path is an instance var

In RestController, `config_path` was declared as a class variable. It is
not. It is an instance variable.
proxyquery: add docstring to `load_config`

This adds parameter docs to RestController.load_config.
proxyquery: fix a premature return in RestController

Fixes a caching-related 500 error in RestController.

RestController.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.)
proxyquery: fix premature return in ApiController

ApiController.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.)