use fog 1.21

Allows us to stop holding credentials in the .fog file
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.
correct spelling
Wording tweak in read me
Merge pull request #178 from alphagov/use_fog_1_21

Use fog 1.21.0 (and vcloud-core 0.11.0)