DOCR: Add support for multiple-registry open beta endpoints

[rak16]

1. Adds a new Registries command to support multiple-registry open beta endpoints, including:
   • create: Create a private container registry
   • delete: Delete a registry
   • docker-config: Generate a Docker auth configuration for a registry
   • get: Get information about a registry
   • list: List all registries in your account
2. Fixed incorrect documentation for Repository sub-commands.
3. Added --registry flag to Repository sub-commands (REST API's already support them).

Testing:

We have multiple-registry open beta endpoints deployed on stage2; to test it out:

1. Run make build
2. Run chmod +x builds/doctl to make sure the build is executable
3. Test against stage2 by setting -u flag:

   builds/doctl registries  \
   -u "https://api.s2r1.internal.digitalocean.com/" \
   -t $DO_TOKEN