Add nsenter container

Because nsenter is not available in
CentOS 6.5, it can be obtained from a
container jpetazzo/nsenter. This is
necessary for running Docker with
the native exec driver.

blueprint fuel-master-docker-host-networking

Change-Id: I1c13abf94ecd41de22f08c633dc1d91377167fda
Merge "Add nsenter container"