Removed from tests: RHEL 7, CentOS 8

[Wayne Workman]

As the title says, RHEL 7 and CentOS 8 have been removed from the daily installation tests.

CentOS 8 had been failing on all branches for some days, something to do with no mirror URLs. @Sebastian-Roth recommended to remove it from the tests, so I went and got this done.

RHEL 7 - there was some problem with getting patches for new AWS Instances. I didn’t spend much time trying to figure it out, as I expect patches to just work when I run yum -y update in AWS. RHEL 7 was also pretty long in the tooth and is nearing it’s ultimate EOL. So, I removed it.