Registry V2 token authentication flow support

Collector now follows the token auth flow for V2 registries that support it, like Docker Hub and Google Container Registry.

For Docker Hub, specify registry as registry-1.docker.io instead of the previous index.docker.io

The new default setting in Collector for the registry type is V2 (override with --registryproto=v1).

v0.5.7

New features:

• Collecting data from images on the local host instead of pulling from a registry.
• Support for Google Container Registry, obtaining docker credentials using GCE metadata or GCE service account JSON key.
• Support for Google Container Registry search endpoint, and specifying repositories with a trailing wildcard character, e.g., myproject/*.