Introduced a SoleInstance cluster and VPC checks
 - Introduced a cluster which contains all the instances in a region
 - Added a Rule to check if instances are in the Virtual Private Cloud
Added VPC check into the SimianArmy Context.
Exclude instances already in an asg cluster from SoloInstances cluster.
Added test for InstanceInVPC. Refactored code to make test easier to run.
Added test for Asg clusters.
Only create solo instances cluster if we have solo instances.
Change from List to Set hence being more specific about the nature of the instances collection.
Fix typo in Constructor to correctly validate soloInstances
- in response to pull request comment
Merge pull request #73 from dipthegeezer/soleinstances

Conformity Check on Virtual Private Cloud Membership and add concept of cluster with instances not in an ASG