[Satellite 6] Foreman-proxy won't start after fresh installation or upgrade

Issue

• After a fresh installation or upgrade using a custom installer answers file, foreman-proxy won't start.

• Satellite installer fails with:

 Execution of '/usr/bin/yum -d 0 -e 0 -y install foreman-release-scl' returned 1: Error: Nothing to do
.... Snip ....
 /Stage[main]/Foreman::Repos::Extra/Package[foreman-release-scl]/ensure: change from 'purged' to 'present' failed: Execution of '/usr/bin/yum -d 0 -e 0 -y install foreman-release-scl' returned 1: Error: Nothing to do
 Could not find a suitable provider for foreman_smartproxy
Installing             Done                                               [100%] [...........................................................................................................]
  Something went wrong! Check the log for ERROR-level output
  The full log is at /var/log/foreman-installer/satellite.log