Add Red Hat Enterprise Linux 8 support for Testgrid WUM builds

[kasunbg]

Description:
We need to provide Red Hat Enterprise Linux 8 support via Testgrid. The requirement is to run wum automated tests against these infras.

Suggested infrastructure parameters:

Name: RHEL-8
OSVersion=8
OS=RHEL
OperatingSystem=RHEL8

[kasunbg]

This infra name is added to tg dev/prod infra-parameters table. Need to add to cloudformation scripts. Specifically, the immediate need is to add this to cloudformation template of ei 6.2.0.