Add support for CentOS 8

[ramereth]

CentOS 8 switched to using dnf instead of yum so the yum-cache-reload ruby_block needs to be updated to use Chef::Provider::Package::Dnf::PythonHelper.instance.restart on CentOS 8.

In addition, CentOS 8 removes the pygpgme as dnf does not require it so let's not emit warnings about it on that platform.

NOTE: This will likely fail due to the fact that the test repo does not have an el8 version of the package referenced in the tests.

Signed-off-by: Lance Albertson lance@osuosl.org

[ramereth]

@jamesgolick @capotej @jmaziarz @ice799 anyway we could get this merged and published soon? Thanks!

[majormoses]

@ramereth I think our best bet might be to pull this into sous-chef maintenance as it does not seem to be actively maintained.

[damacus]

Closed via #66