Add TripleO Heat Template Parameters for Neutron Tenant MTU

Add TripleO Heat Template Parameters for Neutron Tenant MTU

This change adds the TripleO Heat Parameters and Puppet hieradata
to support setting the MTU for Neutron tenant networks. A new
parameter, NeutronTenantMtu is introduced, and this gets used for
the NeutronDnsmasqOptions and in Puppet hieradata.

NeutronTenantMtu is also used in the Puppet hieradata for both the
compute and control nodes. Two values are set:

nova::compute::network_device_mtu

which sets /etc/nova/nova.conf: network_device_mtu = <NeutronTenantMtu>

neutron::network_device_mtu

which sets in /etc/neutron/neutron.conf:
network_device_mtu = <NeutronTenantMtu>

finally, the NeutronDnsmasqOptions parameter becomes a str_format
that maps the NeutronTenantMtu onto the DHCP options,
so a default of 'dhcp-option-force=26,%MTU%' would be formatted to
'dhcp-option-force=26,1300' if NeutronTenantMtu were 1300.

This will set dnsmasq to serve an MTU via DHCP that matches the
NeutronTenantMtu:

/etc/neutron/dnsmasq-neutron.conf:dhcp-option-force=26,1300

Typically, you would change all three of these settings to use small
or jumbo frames in VMs. When using tunneling, NeutronTenantMtu
should be set at least 50 bytes smaller than the physical network
MTU in order to make room for tunneling overhead.

Note that this change does not support setting the MTU on veth
interfaces if veth patches are used to br-int instead of OVS
patches.

Change-Id: I38840e082ee01dc3b6fc78e1dd97f53fa4e63039