Add cnhuser addon.register

Whitelabel partner provide a middleware for addon registration. This
enables the whitelabel partner to handle the authentication and forwards
the manifest content to the cloudControl API.
E.g. cloud&heat have a special login (tenant_name:email), this breaks
the kensa tool.
Introducing addon.register simplifies the onboarding process for addon
providers.
Merge pull request #32 from cloudControl/registeraddon

Add cnhuser addon.register