Ability to check gitsubmodules

[rramsden]

Context

There's quite a bit of duplication across some of our Kaiser projects which check that submodules exist. For example, HATS and HCS both define the following in their kaiser files:

`git submodule status`.lines.each do |line|
  # The git-submodule man page says uninitialized submodules are prefixed with a -
  # but I found this unreliable. While testing I pressed Control-C in the middle of
  # the update command so some submodule would be initialized and others wouldn't.
  # After that, the status command had removed the - for every submodule.
  # Therefore we just check if there's files in the directory instead.
  dir = line.strip.split(' ')[1]
  if Dir.empty?(dir)
    puts "Found uninitialized git submodule '#{dir}'"
    puts "please run 'git submodule update --init --recursive'"
    exit 1
  end
end

Of course this would tie Kaiser to git but that's maybe not a bad thing. If there is a use-case without git than maybe this could be implemented as a helper method e.g. check_git_submodules!