Use fog 1.21.0 (and vcloud-core 0.11.0)

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.
Merge pull request #40 from alphagov/use_fog_1_21

Use fog 1.21.0 (and vcloud-core 0.11.0)