add option to run K with nailgun

@yilongli @osa1 please review

This pull request does not actually activate nailgun by default. In order to use nailgun, you must first set the K_NAILGUN environment variable. Once this is committed, I will experiment with the jenkins job to try to get it to run with nailgun.
Could nailgun live in the `k-distribution` to keep the `kernel` small? Running the entire `ktest` suite requires the k-distribution anyhow.

if yes and we agree it would be a good idea, I still think the pull request should me merged. The speed advantage is too important to postpone.