Use fog 1.21.0 (and vcloud-core 0.11.0)

This allows the user to stop holding credentials in a .fog file.

* Note this relies on releasing of the vcloud-core gem 0.11.0 before being pulled down to master (https://github.com/alphagov/vcloud-core/pull/15)

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 further detail.