Found a few codec inconsistencies between JSON encoders and decoders

- Fix EtherType inconsistency in coding VLAN_PUSH and MPLS_POP instructions
- Fix group id inconsistency in coding a Group instruction
- Fix Group identifiers to be treated as unsigned integers in GroupCodec
- Add a JSON decoder for FlowEntry

Change-Id: Id80b9e009550e3b71b9274adb116e5179391fa66
(cherry picked from commit 494f586853c1a87ba62f20b477e211f996baded5)
diff --git a/core/common/src/test/resources/org/onosproject/codec/impl/simple-group.json b/core/common/src/test/resources/org/onosproject/codec/impl/simple-group.json
index 865ae23..2229372 100644
--- a/core/common/src/test/resources/org/onosproject/codec/impl/simple-group.json
+++ b/core/common/src/test/resources/org/onosproject/codec/impl/simple-group.json
@@ -2,7 +2,7 @@
   "type": "ALL",
   "deviceId": "of:0000000000000001",
   "appCookie": "0x1234abCD",
-  "groupId": "1",
+  "id": "1",
   "buckets": [
     {
       "treatment": {