Add handlers for client network MTUs

Refactors some of the code in VXNetworkInterface to handle MTUs in a
more streamlined fashion. Also fixes a bug whereby bridge client
networks were being explicitly given the cluster dev MTU which might not
be correct. Now adds support for this option explicitly in the configs,
and defaults to 1500 for safety (the standard Ethernet MTU).

Addresses #144

daemon-common/zkhandler.py

@@ -466,7 +466,7 @@ class ZKHandler(object):
 #
 class ZKSchema(object):
     # Current version
-    _version = 5
+    _version = 6
 
     # Root for doing nested keys
     _schema_root = ''
@@ -595,6 +595,7 @@ class ZKSchema(object):
         'network': {
             'vni': '',  # The root key
             'type': '/nettype',
+            'mtu': '/mtu',
             'rule': '/firewall_rules',
             'rule.in': '/firewall_rules/in',
             'rule.out': '/firewall_rules/out',