VOL-270: Investigate and support clustering support for xPON protos

Ensuring global-scope for:
 * ChannelgroupConfig
 * ChannelpairConfig
 * ChannelpartitionConfig
 * ChannelterminationConfig
 * VOntani
 * Ontani
 * VEnets

Making relevant changes into itests for xPON

Adding xPON test cases for test_dispatcher and global rest apis

Change-Id: I8b1ed1cb314810886cd25bee82ffc741f7fe6fcf
diff --git a/common/utils/id_generation.py b/common/utils/id_generation.py
index eedf434..59535b8 100644
--- a/common/utils/id_generation.py
+++ b/common/utils/id_generation.py
@@ -50,6 +50,14 @@
     assert id and len(id) == 16
     return id[:4] == BROADCAST_CORE_ID
 
+def create_empty_broadcast_id():
+    """
+    Returns an empty broadcast id (ffff000000000000). The id is used to
+    dispatch xPON objects across all the Voltha instances.
+    :return: An empty broadcast id
+    """
+    return '{}{}'.format(BROADCAST_CORE_ID, '0'*12)
+
 def create_cluster_id():
     """
     Returns an id that is common across all voltha instances.  The id