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.
The metadata service often times will provide a deterministic hostname for the instance. If the user provides a hostname in their cloud-config, it would have been overridden by the metadata. When the order is flipped, the user_data is the last to be applied and therefore the highest precedent. Ultimately, this needs to be reworked slightly so that we only apply the settings once.