Add MTU to lxc conf to make host and container MTU match

If you are using jumbo frames the host side of the veth was being set to
1500.  Docker would set the MTU of the container side of the veth to 9001.
This would lead to a situation in which the two sides of the veth had
different MTU sizes causing issues in network traffic.

Docker-DCO-1.1-Signed-off-by: Darren Shepherd <darren.s.shepherd@gmail.com> (github: ibuildthecloud)
make docker build -rm=true default #4292

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
Merge pull request #4292 from cpuguy83/4291_make_rm_default_for_build

make docker build -rm=true default
Merge pull request #4282 from ibuildthecloud/jumbo-frames

Add MTU to lxc conf to make host and container MTU match