Add support for --runtime

[shishir-a412ed]

Currently containerd-driver gives the ability to set the runtime at the plugin level

It doesn't allow you to choose runtime at the job level similar to this

Add a flag --runtime to allow selecting runtime per job (Assuming runtime is installed and available on the nomad client node)
This will allow users to run containers with multiple runtimes e.g. runc and runsc (gVisor) on the same node.

[t-hale]

Are you still working on this in #83 ?

[shishir-a412ed]

@t-hale Sorry I am not actively working on this project right now, due to other competing priorities.
Feel free to pick it up if you like. I ll try to help with the review.