cmdlib.sh: add env var to sleep before killing tail

I can't reproduce this locally, but I have a suspicion that `tail` can
exit too quickly in some circumstances, causing truncated output:

openshift/os#1498 (comment)
coreos#3785 (comment)

Rather than having an unconditional `sleep`, let's make it easier to
test that theory by having an env var we can use to make it optional.
Then we'll test that in CI.

Mid-term, I'd like to revert 79b15c8 soon so we can go back to
virtio-serial which is just so much cleaner.

src/cmdlib.sh

@@ -845,6 +845,10 @@ EOF
     fi
     rc="$(cat "${rc_file}")"
 
+    # XXX: this is for debugging temporarily
+    if [ -n "${TAIL_SLEEP:-}" ]; then
+        sleep "${TAIL_SLEEP}"
+    fi
     # cleanup tail before nuking dir containing file it's following
     kill "$tail_pid"