Add Puppet 8 support

[bastelfreak]

No description provided.

[ekohl]

Looks like a legitimate failure:

OpenSSL::PKey::PKeyError:
  pkeys are immutable on OpenSSL 3.0
# ./spec/unit/puppet/provider/ssl_pkey/openssl_spec.rb:113:in `generate_key!'
# ./spec/unit/puppet/provider/ssl_pkey/openssl_spec.rb:113:in `block (2 levels) in <top (required)>'
# ./spec/unit/puppet/provider/ssl_pkey/openssl_spec.rb:112:in `block in <top (required)>'
# ./spec/unit/puppet/provider/ssl_pkey/openssl_spec.rb:7:in `<top (required)>'

[ekohl]

On Fedora I needed #189, but for me it refuses to generate DSA keys. Not entirely sure if that's an OpenSSL policy.

[ekohl]

Rebased to include #223