[terraform-ibm-schematics-agent] Follow up actions after initial release

[ocofaigh]

• Document any current known issues (e.g. provider limitations, supported agent versions, supported agent types)
• Add validation for agent version - we should probably only allow what the module tests
• infra_type var supports ibm_openshift but I'm hearing Openshift is not support yet? If not, then remove it as an allowed value
• can we update this code comment with a link to a tracking provider issue
• add Required IAM access policies to readme (currently its empty)