Deprecate metadata_neutron_* configuration settings

Commit 5cacad3508570ce70b1f9ef620e0508169687fda created a
neutron configuration section. Move all of the metadata
neutron settings to that section.

DocImpact

The table below has the changes:

+-------------------------------------+------------------------------+
| 'DEFAULT' Section                   | 'neutron' Section            |
|-------------------------------------|------------------------------|
| service_neutron_metadata_proxy      | service_metadata_proxy       |
| neutron_metadata_proxy_shared_secret| metadata_proxy_shared_secret |
+-------------------------------------+------------------------------+

Change-Id: Id1b8b387e028c7752729f5c5b373c87397f7577a
Merge "Deprecate metadata_neutron_* configuration settings"