Default to using ssd storage for ec2 instances
Optional Storage constraint no longer a pointer
Merge pull request #922 from wallyworld/aws-ssd-root-disk

Default to using ssd storage for ec2 instances

Fixes: https://bugs.launchpad.net/juju-core/+bug/1381009

When an EC2 image is searched for in simplestreams, the preference is to use one with SSD storage. If none is found, an image with EBS storage is used if found.

Also cleans up the image filtering which was incorrectly hard coding the storage type to search for.