Remove Docker-specific code from manager

This is for tracking @Pradeep's comment in https://review.openstack.org/#/c/392886/ :

  I have a basic concern, i may be wrong,
  1) IMHO manager should not handle errors and print errors only for docker runtime, i mean to say it should be something dependent on container runtime i.e. container driver, suppose i use rocket driver then this message doesn't make sense,
  2) Can we have different exceptions based on operation like ImagePullError or ContainerCreateError or something most appropriate, currently only DockerError is there.

In summary, in file zun/compute/manager.py, there are logic to handle docker-specific exception. However, manager should be generic so it should not handle runtime-specific error. This is because we are going to introduce another runtime in the future.

We need a fix to move all docker-specific code from manager.py to the docker driver.