neutron_metadata_proxy_shared_secret should not be written to log file

This patch sets secret=True for the following config option
neutron_metadata_proxy_shared_secret

Conflicts:

	nova/api/metadata/handler.py

Change-Id: I381ea9e89f76beac11b03def6e3ccfee2a55e48f
Closes-bug: #1288463
(cherry picked from commit fa73a5d037471dbec11893ad83bd93276968b9af)
Merge "neutron_metadata_proxy_shared_secret should not be written to log file" into stable/havana