aws_region is mandatory for AWS cluster but not validated

[mpagot]

I'm creating a deployment with playbook using cluster role from this repo: SUSE/qe-sap-deployment#202

I noticed that, if I forgot to provide aws_region, the role fails as the variable is needed by task Ensure config file has cluster block.

That's fine but:

• it is not properly documented in the readme, in the Required variables for AWS section of

  community.sles-for-sap/roles/cluster/README.md

  Line 144 in 00c5182

  ### Required Variables for AWS

• it is not validated by task Ensure that AWS specific variables are supplied

[marcelmamula]

PR was merged.