use fog 1.21

This allows the user to stop holding credentials in a .fog file.

This includes the raw fog changes only. To use it you'll need to get a
session token, by externally logging in, and set that as FOG_VCLOUD_TOKEN
ENV var.
see https://github.com/fog/fog/pull/2705 for futher detail.

There are further changes required in other vcloud repos where
they depend on fog directly before this feature can be fully adopted.
Updating README to reflect new way to submit credentials.

With fog 1.21.0 you can now log on externally and provide your vcloud
token to fog.
Merge pull request #15 from alphagov/use_fog_1_21

Use fog 1.21.0