Stop all neutron-ns-metadata-proxy with stop_neutron

Process name is actually python therefore neutron-ns-metadata-proxy
pattern didn't match wanted process.

Closes-bug: #1269982
Change-Id: Ib4439b0d32f103253b461841fa903c65763ff280
(cherry picked from commit 1f76328027bb5cee0b0ea7077f4c59c919f1c4ae)