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.
Ah yeah, we needed it when it was a string cause it's optional.