Eureka should log the total number of instances

Eureka client would only log the number of UP instances but with the
deceiving message:

logger.debug("The total number of instances in the client now is {}",
                totInstances);
Fix appsHashCode not getting updated

appsHashCode was not getting updated under 2 circumstances.

First when a BackupRegistry is used at all. Second when
fetchRegistry is called. Most notably when this is called
by the CacheRefreshThread.
Merge pull request #130 from Knewton/fix-instance-count-log

Fix Eureka instance count and status logging
Adding config override in eureka-client to force registry refresh to only pick up
data for a single app (specified by the config).
Merge branch 'single-app-registry-refresh' of https://github.com/qiangdavidliu/eureka into single-app-registry-refresh

Conflicts:
	eureka-client/src/test/java/com/netflix/discovery/AbstractDiscoveryClientTester.java
	eureka-client/src/test/java/com/netflix/discovery/DiscoveryClientRegistryTest.java