Add support for 4.18 and adjust tests with RH versions supported

[vbontempi]

Version 4.18 for openshift cluster is released

https://cloud.ibm.com/docs/openshift?topic=openshift-openshift_versions#os-openshift-with-coreos

RHEL_8_64 version is not available anymore on this version, so we need to adjust the tests to change the workers OS for 4.18

[Vipin654]

PR has been merged. #715

OCP 4.18 is now supported in the module. Default value in DA is still 4.17 because ibmcloud ks versions still return 4.17. It will be changed later.