[#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 don't think so because it's one huge string: https://gist.github.com/dcarley/7b3566931b1e77ac3481

We could just throw away the error, on the basis that the password doesn't exist if we can't access it.
```diff
diff --git a/lib/vcloud/fog/login.rb b/lib/vcloud/fog/login.rb
index 64ccb6e..c94798b 100644
--- a/lib/vcloud/fog/login.rb
+++ b/lib/vcloud/fog/login.rb
@@ -20,7 +20,12 @@ module Vcloud
       private

       def self.check_plaintext_pass
-        pass = ::Fog.credentials[FOG_CREDS_PASS_NAME]
+        begin
+          pass = ::Fog.credentials[FOG_CREDS_PASS_NAME]
+        rescue ::Fog::Errors::LoadError
+          return
+        end
+
         unless pass.nil? || pass.empty?
           raise "Found plaintext #{FOG_CREDS_PASS_NAME} entry. Please set it to an empty string"
         end
```
In which case we get the nicer error message from instantiating `Fog::Compute::VcloudDirector`:
```
➜  vcloud-core git:(vcloud_login_token_tool) ✗ b ./bin/vcloud-login
vCloud password:
Missing required arguments: vcloud_director_username, vcloud_director_host
```