Update README to reference new vcloud-login
Update README to reference new vcloud-login

Simplify the instructions for fetching a session token by using the new
vcloud-login utility. This will also need to be applied, or better
referenced, by the other projects once merged.

gds-operations/vcloud-core,UKHomeOffice/vcloud-core,gds-operations/vcloud-core,UKHomeOffice/vcloud-core