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 further detail.
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.
add additional details about using vcloud token in README
Remove bundle exec from example

Its main usage as a gem doesn't need it.
Merge pull request #59 from alphagov/use_fog_1_21

Use fog 1.21.0 (and vcloud-core 0.11.0)