[#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 
It doesn't appear so;

```ruby
» bundle exec pry
[1] pry(main)> require 'vcloud/fog/login'
=> true
[2] pry(main)> Vcloud::Fog::Login.check_plaintext_pass
Fog::Errors::LoadError: Missing Credentials

To run as 'default', add the following to your resource config file: /Users/matt/.fog
An alternate file may be used by placing its path in the FOG_RC environment variable

#######################################################
# Fog Credentials File
#
# Key-value pairs should look like:
# :aws_access_key_id:                 022QF06E7MXBSAMPLE
:default:
  :aws_access_key_id:
  :aws_secret_access_key:
<snip>
```