add support for RHEL8 and puppet 6 and 7

Author: vchepkov

metadata.json

@@ -12,15 +12,16 @@
       "operatingsystem": "RedHat",
       "operatingsystemrelease": [
         "6",
-        "7"
+        "7",
+        "8"
       ]
     }
   ],
   "dependencies": [],
   "requirements": [
     {
       "name": "puppet",
-      "version_requirement": ">= 3.8.0 < 6.0.0"
+      "version_requirement": ">= 3.8.0 < 8.0.0"
     }
   ]
 }