Did the following changes:

1) Provided a default availability zone -> region mapping for ec2 so that every client does not have to provide the same. This can be overridden by a property per region.
2) Fixed a bug in DiscoveryClient.getInstancesByVipAddress() for a region. For invalid VIPs, the null check was wrong.
Merge pull request #36 from NiteshKant/master

Bug fix & default az->region mapping