Allow specification of non-default mtu for instances:

When using GRE tunelling, large packets are sometimes corrupted,
unless a smaller mtu is used.

This change allows setting a non-default MTU in var `neutron_dhcp_mtu`,
which is passed to dnsmasq, and provided to instances when they DHCP.
Merge pull request #22 from blueboxgroup/mtu

Allow specification of non-default mtu for instances: