Fix pid file location to avoid I->J changes that break metadata

Fix pid file location to avoid I->J changes that break metadata

Changes in commit 7f8ae630b87392193974dd9cb198c1165cdec93b moved
pid files handled by agent/linux/external_process.py from
$state_path/external/<uuid>.pid  to $state_path/external/<uuid>/pid
that breaks the neutron-ns-metadata-proxy respawn after upgrades
becase the l3 or dhcp agent can't find the old pid file so
they try to start a new neutron-ns-metadata-proxy which won't
succeed, because the old one is holding the port already.

Closes-Bug: #1376128
Change-Id: Id166ec8e508aaab8eea35d89d010a5a0b7fdba1f