Add NeutronGlobalPhysnetMtu to neutron-base.yaml

This adds an option to configure the global_physnet_mtu in
neutron.conf. This controls the base MTU for the physical network
which is used as a basis for the new auto calculated overlay network
MTU's.

For flat and VLAN tenant networks there has been a subtle breakage
in TripleO in that users of those network types may now need to
manually set NeutronGlobalPhysnetMtu to something less than the
physical network to imitate the previous behavior of the
now removed NeutronTenantMtu setting (removed in git
2a64b67cef). Previously users of
say a vlan tenant network in TripleO would automatically get
the tenant network set to 1400.

Change-Id: I25c2435feb373e11e4fc3a2821db0c96211468e0

puppet/services/neutron-base.yaml

@@ -48,6 +48,15 @@ parameters:
     description: >
         Remove configuration that is not generated by TripleO. Setting
         to false may result in configuration remnants after updates/upgrades.
+  NeutronGlobalPhysnetMtu:
+    type: number
+    default: 1500
+    description: |
+        MTU of the underlying physical network. Neutron uses this value to
+        calculate MTU for all virtual network components. For flat and VLAN
+        networks, neutron uses this value without modification. For overlay
+        networks such as VXLAN, neutron automatically subtracts the overlay
+        protocol overhead from this value.
   ServiceNetMap:
     default: {}
     description: Mapping of service_name -> network name. Typically set
@@ -87,3 +96,4 @@ outputs:
         neutron::host: '"%{::fqdn}"'    #NOTE: extra quoting is needed
         neutron::db::database_db_max_retries: -1
         neutron::db::database_max_retries: -1
+        neutron::global_physnet_mtu: {get_param: NeutronGlobalPhysnetMtu}