Restablish networking upon container reboot

Currently, whenever a container is rebooted, it will
lose its IP address.

This happens because a container's ID and PID will change, while the
namespaces are mapped to both.

This fix unplugs and replugs networking between stopping and
starting of the container.

Closes-Bug: #1256256

Change-Id: I583926e43c832605c77852205b4e2df8ce9746ed
Merge "Restablish networking upon container reboot"