Switch from centos8 testing to almalinux8

[archanaserver]

theforeman/foreman-infra#2090

[ekohl]

EL7 fails on a weird reason:

Collecting mock<4.0.0 (from -r test-requirements.txt (line 2))
  Could not fetch URL https://pypi.python.org/simple/mock/: There was a problem confirming the ssl certificate: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:618) - skipping
  Could not find a version that satisfies the requirement mock<4.0.0 (from -r test-requirements.txt (line 2)) (from versions: )
No matching distribution found for mock<4.0.0 (from -r test-requirements.txt (line 2))

But EL8 passes, so I'm going to merge this. Thanks!