Improve mib_dir setting consistency (#12069)

yaml os discovery now respects the os mib_dir definition
mib_dir now only allows strings, not arrays
changed schema validation to account for that
consolidated dell mibs, no reason for them to be spread out

misc/os_schema.json

@@ -347,7 +347,9 @@
         "ifname": {
             "type": "boolean"
         },
-        "mib_dir": {"$ref": "#/definitions/string_group"},
+        "mib_dir": {
+            "type":  "string"
+        },
         "good_if": {
             "type": "array",
             "items": {