Add feature to update hosts and images

By way of simulated help text, the feature should behave as follows:

Usage:
  nest update [options]

Options:
  -b, [--boot-env]  # Perform the update in the alternate A/B boot environment
  -d, [--dir=DIR]   # Update a specific directory exclusive of boot environments
  -k, [--kernel]    # Also update the kernel
  -y, [--yes]       # Update packages without asking

Description:
  Perform a traditional package-based update and run Puppet.

In the case of a failed update, the boot environment can be mounted (nest beadm mount) and continued with nest update -d /mnt/BOOTENV.