Use salt's master `clean_proc` routine.

On Fedora 20, we hit an issue, at least, similar to #3618.
While debugging it, I've found that the runtests process keeps eating
memory.

Using `clean_proc` has at least, allowed the runtests to finish without
triggering #3618.
Merge pull request #12106 from s0undt3ch/develop

Use salt's master `clean_proc` routine.