vxlan default ml2 tenant network type

ovs vxlan become supported on most distribution and provides
isolated `multi tenant` networking without special `vlan` aware switch.

In single node deployment you will just see the br-tun ovs bridge
created.

In multi-node setup you might need to adjust the MTU settings.

If your physical switch supports >=1550 MTU size
 probably you will not see any issue.

If your guest image honors the MTU size offered via dhcp,
you can adjust your dnsmask settings with the correct (likely 1450 byte)
MTU size.

cirros (udhcp) does not honors these setting, you might need to
set lower MTU size on br-ex and/or adjust network_device_mtu option
in your local.conf.

The default changed, because it will be used with the multi-node neutron
jobs.

If you want the original behavior for whatever reason add these to your
`local.conf`:
ENABLE_TENANT_TUNNELS=False
Q_ML2_TENANT_NETWORK_TYPE=local

Change-Id: Id33ff0eca44905b3996618f1035ad984a6819b5b
Merge "vxlan default ml2 tenant network type"