Add pack set-default-registry command

[elbandito]

Signed-off-by: Travis longoria.public@gmail.com

Summary

This PR introduces a new pack command to allow the user to set the default registry name via pack set-registry.

Output

New

❯ pack
CLI for building apps using Cloud Native Buildpacks

Usage:
  pack [command]

Available Commands:
  set-default-registry              Set default registry

❯ pack set-default-registry --help
Set default registry

Usage:
  pack set-default-registry <name> [flags]

Examples:
pack set-default-registry myregistry

Flags:
  -h, --help   Help for 'set-registry'

Global Flags:
      --no-color     Disable color output
  -q, --quiet        Show less output
      --timestamps   Enable timestamps in output
  -v, --verbose      Show more output

Documentation

• Should this change be documented?
  • [ X ] Yes, see #___
  • No

Related

Resolves #745

[dfreilich]

2 minor nits, but overall looks great!

[dfreilich]

One more nit: can we move the cmd.Example to be part of the definition of the cmd?

[elbandito]

@jromero and @dfreilich I made changes per comments. Can I get a final look and blessing?

Changes made