Docker Driver doesn't respect CPU limit

The docker driver doesn't respect the CPU settings of the configured
flavor.

Docker/LXC supports only relative cpu alloaction. The relative
cpu alloaction doesn't prevent that a container can use all CPUs but it
can at least insure that a container with more CPUs gets more CPU
cycles.

Change-Id: I7b299c1afd9c1269a07da4079c7272955a67822a
Closes-Bug: #1256264