Added Ec2 Placement Group support

Added support for EC2 Placement Groups by following the pattern for added support of VPC subnets and security group Id's (https://github.com/saltstack/salt-cloud/pull/585/files#r4282815).

Info on Ec2 Placement Groups: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-groups.html

Placement Group API reference: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-RunInstances.html