Add functions for manipulating VM tags

Adds tags to schema (v3), to VM definition, adds function to modify
tags, adds function to get tags, and adds tags to VM data output.

Tags will enable more granular classification of VMs based either on
administrator configuration or from automated system events.

daemon-common/zkhandler.py

@@ -466,7 +466,7 @@ class ZKHandler(object):
 #
 class ZKSchema(object):
     # Current version
-    _version = 2
+    _version = 3
 
     # Root for doing nested keys
     _schema_root = ''
@@ -576,6 +576,7 @@ class ZKSchema(object):
             'meta.migrate_method': '/migration_method',
             'meta.node_selector': '/node_selector',
             'meta.node_limit': '/node_limit',
+            'meta.tags': '/tags',
             'migrate.sync_lock': '/migrate_sync_lock'
         },
         # The schema of an individual network entry (/networks/{vni})