add oracle normalize os spec test

Author: nasark

spec/models/operating_system_spec.rb

@@ -15,6 +15,8 @@
       "linux debian"               => "linux_debian",
       "redhat coreos"              => "linux_coreos",
       "rhcos-4.11.2"               => "linux_coreos",
+      "ol_8x64"                    => "linux_oracle",
+      "oracle"                     => "linux_oracle"
     }.each do |image, expected|
       it "normalizes #{image}" do
         expect(described_class.normalize_os_name(image)).to eq(expected)