[#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 
This still feels wrong and it triggers a very verbose error if you haven't passed a `FOG_CREDENTIAL`. But I can't see a better way to do this, besides invoking `Fog::Compute::VcloudDirector.new` and peeking into `instance_variable_get(:@vcloud_director_password)`.