Update Fog Credentials README section

This updates the Fog Credentials section to be equivalent to the
instructions in vcloud-core and vcloud-tools.

It is more concise and clear about how folk should handle FOG_VCLOUD_TOKEN,
including a strong recommendation not to use vcloud_director_password.
Merge pull request #16 from alphagov/credentials_in_readme

[#69702304] Update Fog Credentials README section