Fix stop_neutron metadata agent function

Currently, stop_neutron fails in Jenkins because it kills itself.
This patch ensure we kill only neutron metadata agent, and not the awk
process in itself.

Change-Id: I0f1f296b639096f08c7018cd1daebfcd9a054d01
Closes-bug: #1269982
(cherry picked from commit 04f6dc24a7845ee139977fa5b0c5e53aad8e99bd)
Merge "Fix stop_neutron metadata agent function" into stable/havana