metadataService: Check both ec2 and openstack urls more explicitly

Remove the root url parameter for -from-metadata-service since this
is a guarenteed value. Additionally, check for both ec2 and openstack
urls for the metadata and userdata. Fix a bug with the -from-url
option and a panic on an empty response.
We should be able to merge lists (i.e. ssh keys) and allow user data to override metadata. What do you think?