docker containerizer should not delete container until gc runs

When a task using the docker containerizer fails docker rm is immediately called and the container is removed. I argue the container should not be removed until the garbage collector runs. This gives users an opportunity to diagnose what went wrong. MESOS-1652 will help with that, but there are other use cases where I want to inspect the state of the container that involve looking at files other than what was produced on stdout and stderr.

tnachen discussed this on IRC. See http://wilderness.apache.org/channels/?f=mesos/2014-07-31#1406842111