cloudconfig: Process metadata before userdata

This gives the options in userdata a higher precedence over metadata.
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.
Merge pull request #159 from crawford/metadata

metadataService: Check both ec2 and openstack urls more explicitly