Make it easier to implement e2e specs

[nickthecook]

There are a few things that make implementing an e2e spec more work and less DRY than it should be.

This is an example:

RSpec.describe "ssh key with passphrase var" do
  include_context "ops e2e"

  before(:all) do
    Dir.chdir(__dir__)

    remove_untracked_files

    @output, @output_file, @exit_status = run_ops("../../../../bin/ops up")
  end

  # actual tests goes here
end

Things that could be improved:

• doing a chdir: must be done in every spec, correctly, or the tests will run with a) no ops.yml or b) worse: the wrong ops.yml
• removing untracked files: must be done in every spec (so far; maybe someday a spec won't need it)
• setting local variables: I'm not sure how legit this is to do in an rspec spec; maybe there's a better way
• knowing the path back to the ops installation: maybe a method in "ops e2e" context could handle this, if you pass an array like ["up", "my_action", "down"]

Basically, having to copy and paste this block to every spec and tweaking the path to bin/ops is not ideal.

[nickthecook]

This allows multiple ops commands to be tested in a single spec file, but it's a total hack:

https://github.com/nickthecook/ops/blob/4dc85856e1868f906730ffb07aebfa4cac46bae9/spec/e2e/forwards/e2e_spec.rb#L12