Add Puppet 8 support

[bastelfreak]

also contains #1125

[ekohl]

CI needs to be updated for it to run tests on Puppet 8.

[ekohl]

Needs #1133 so it actually runs CI.

[ekohl]

I think all fixes are in place. Please rebase.

[ekohl]

I did notice there were deprecation warnings about the ERB template:

puppet-foreman/templates/settings.yaml.erb

Line 2 in 93dbe6a

ERB.new(File.read(File.expand_path("_header.erb",File.dirname(file))), nil, nil, "@header").result(binding) -%>

This needs to use keyword arguments now (and is repeated in multiple files).

[bastelfreak]

@ekohl rebased on #1125

[ekohl]

Test failure is relevant. I think the exception needs to be changed to a general Puppet::Error instead of a ParseError.

[ekohl]

I think #1139 should be complete. I've opted to drop the deprecated non-namespaced functions instead of trying to fix them.