Increase MTU on interfaces used for GRE tunnels

Increase MTU by 46 bytes to 1546 so GRE encapsulation won't reduce the path
MTU to 1454. This workarounds issues where some sites expect a PMTU of 1500
and ignore the "Too big - needs fragmentation" ICMP. Also if additional
NATing is used, it might not translate the private address inside the ICMP.

Vagrant writes /etc/network/interfaces at boot, so using it here instead of
Ansible to avoid conflict.