Update Fog Credentials README section
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.

gds-operations/vcloud-launcher,UKHomeOffice/vcloud-launcher,UKHomeOffice/vcloud-launcher,gds-operations/vcloud-launcher,UKHomeOffice/vcloud-launcher,gds-operations/vcloud-launcher