Deprecate neutron_* configuration settings

Create a new section in the configuration file called 'neutron'.
Move all of the neutron_* configuration settings to this section.

DocImpact

The table below has the changes:

+---------------------------------+-------------------------+
| 'DEFAULT' Section               | 'neutron' Section       |
|---------------------------------|-------------------------|
| neutron_url                     | url                     |
| neutron_url_timeout             | url_timeout             |
| neutron_admin_username          | admin_username          |
| neutron_admin_password          | admin_password          |
| neutron_admin_tenant_id         | admin_tenant_id         |
| neutron_admin_tenant_name       | admin_tenant_name       |
| neutron_region_name             | region_name             |
| neutron_admin_auth_url          | admin_auth_url          |
| neutron_api_insecure            | api_insecure            |
| neutron_auth_strategy           | auth_strategy           |
| neutron_region_name             | region_name             |
| neutron_ovs_bridge              | ovs_bridge              |
| neutron_extension_sync_interval | extension_sync_interval |
| neutron_ca_certificates_file    | ca_certificates_file    |
+---------------------------------+-----------------------=-+

UpgradeImpact
The changes are backward compatible.

Change-Id: I3308be33f60c4add3e2e78b75f238059a9b3d876
Merge "Deprecate neutron_* configuration settings"