[#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 
I see. Rather than surpressing `LoadError`, could we match the 'Missing Credentials' [message](https://gist.github.com/dcarley/7b3566931b1e77ac3481) and raise a more user-friendly error? Maybe that would be too brittle?