Duplicate name-value env-vars in '-e' option of docker run

docker run provides the option '-e' to pass environment variables.

In src/docker/docker.cpp->Docker::run(), environment vars are picked up from passed in 'env' as well as from 'commandInfo.environment().variables()'.
But with git hash 391e72b9 (https://reviews.apache.org/r/31889, MESOS-2463), 'env' is already populated with variables() [see src/containerizer/containerizer.cpp->executorEnvironment()] as a result of which env vars are duplicated (the ones in Environment::Variable) in the docker run command.