Support starting agent containers in privileged mode

[varshavaradarajan]

This will help in using a docker in docker image to run docker related tasks using elastic agents.

[akshaymayekar]

@varshavaradarajan It seems that starting docker service in privilege mode is not yet supported in docker swarm due to security and implementation complexities involved. Here is the open issue in docker github for supporting privilege mode for docker swarm https://github.com/docker/swarmkit/issues/1030

https://forums.docker.com/t/privileged-containers/36935