WIP - Metadata

[Rucknar]

Changes:

• Moved to sourcing information from metadata service
• No longer requires API Access Key + Secret
• Simplified codebase to match the simpler metadata service data
• Moved to constmetrics, dropped locking
• Moved to multi-stage builds
• Made use of go-common

Still to do:

• Add back in health status for services & stacks, requires changes to metadata service
• Debug logging, fix handling and passing around the go-common logging
• Add back in state for hosts, requires changes to metadata service
• Improve how we handle the rancher-metadata URL and version
• Resolve 'built-in' metrics through the go-common router. Currently initialised but not appearing.
• If possible, obtain possible states from rancher directly or from a standard library

[Rucknar]

@flock3 Added you as a reviewer, not yet finished but would appreciate any feedback