Fix Redis LRU cache default configuration backwards compatibility

Hi, 

   I found this issue when configuring a production registry based on docker image 0.7.3. The current master sample_config.yml might cause compatibility issue when this branch is released.

   The Redis cache YAML configuration should be left undefined value by default.

   Based on [cache.py line 40](https://github.com/dotcloud/docker-registry/blob/d69590228b03b4d54a871bda2a7557d3298c7528/docker_registry/lib/cache.py#L40), the cache is enabled when host is defined. 

   I think the issue was introduced in commit https://github.com/dotcloud/docker-registry/commit/713d7bc72879f38e88d7859a8c5c7ea46099b5b9 with default redis information added.
   Then it was incorrectly patched by commenting out the configuration to fix travis test in commit https://github.com/dotcloud/docker-registry/commit/e014c53ed3c329fa46300b37311672871aef1f48


   I also added two environment variables `CACHE_REDIS_DB` and `CACHE_LRU_REDIS_DB`