[#68989754] Add vcloud-login utility for session tokens

As described by the --help argument:

    Usage: ./bin/vcloud-login [options]

    Utility for obtaining a Fog vCloud Director session token. It will output a
    shell `export` command that can be consumed with:

        eval $(FOG_CREDENTIAL=example ./bin/vcloud-login)

    It requires a Fog credentials file (e.g. `~/.fog`) with the host and user
    set, but the password set to an empty string. The password can either be
    entered interactively or piped in, for example from an environment variable:

        printenv PASSWORD_VAR | FOG_CREDENTIAL=example #{$0}

This should serve to consolidate the various scripts other teams have needed to
create in order to follow the advice about not storing plaintext passwords in
the fogrc file.

---

I'm going to deal with preventing plaintext password use in a separate PR - but we might as well bundle them both in the same release.

/cc @annashipman @mattbostock 
the public/private etc keywords only apply to methods defined without a specific receiver (ie `def foo...`, and not `def self.foo`, or `def Foo.foo`).  A way round this for class methods is to do something like:

```ruby
class << self
  def token(pass)
    #...
  end

  def token_export(*args)
    return "export #{TOKEN_ENV_VAR_NAME}=#{token(*args)}"
  end

  private

  def check_plaintext_pass
    # ...
  end

  def get_token(pass)
    # ...
  end
```

or use [`private_class_method`](http://apidock.com/ruby/Module/private_class_method)

See http://stackoverflow.com/questions/4952980/creating-private-class-method for a discussion of this.