Inherit Intent Key on compilation

- Added Intent#key() method description
- Inherit key field from parent Intent during Intent
  compilation process
- Added assertion to existing unit tests

Change-Id: Iff85c5ec448b3f378957b7a20af865ad96cc3216
diff --git a/core/net/src/main/java/org/onosproject/net/intent/impl/compiler/MultiPointToSinglePointIntentCompiler.java b/core/net/src/main/java/org/onosproject/net/intent/impl/compiler/MultiPointToSinglePointIntentCompiler.java
index f9b236f..cac5d68 100644
--- a/core/net/src/main/java/org/onosproject/net/intent/impl/compiler/MultiPointToSinglePointIntentCompiler.java
+++ b/core/net/src/main/java/org/onosproject/net/intent/impl/compiler/MultiPointToSinglePointIntentCompiler.java
@@ -119,6 +119,7 @@
 
         Intent result = LinkCollectionIntent.builder()
                 .appId(intent.appId())
+                .key(intent.key())
                 .treatment(intent.treatment())
                 .selector(intent.selector())
                 .links(Sets.newHashSet(links.values()))