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

Change-Id: I381ea9e89f76beac11b03def6e3ccfee2a55e48f
Closes-bug: #1288463
Merge "neutron_metadata_proxy_shared_secret should not be written to log file"