Refactor location-resolvers

- previously had two newLocationFromString methods, where second
  was only ever called by first. So deleted second method.
location-resolver: allow any properties in spec

- for byob, localhost and jcloudsByon, don't enforce just a few
  properties; instead accept any property
Rename LocalhostResolver to LocalhostLocationResolver
Fix CentOS 6.3 VM used in AWS tests

- as requested by Zaid because the previous one had problems with
  yum repos; this is an official one from CentOS
Adds AbstractLocationResolver
Rename JcloudsResolver to JcloudsLocationResolver
This closes #104