Attempting to clarify the conditonal SSL configuration

[lurraca]

While paving our new GCP project with this terraform template we were confused as whether we needed to actually include the properties for SSL certificaties in our terraform.tfvars since they are marked as optional, and while the description of the properties state that we needed one or the other we felt that the presentation of that information made it easy to skim and ignore.

Changes:

• Mark SSL properties as conditionally required instead optional
• Fix typo on create_iam_service_account_members property description to show the default value.

Co-authored-by: Scott Muc smuc@pivotal.io

[zachgersh]

@lurraca / @scottmuc, great change thank you so much. If you see any other README.md changes please let us know.

❤️ @zachgersh && @crhntr