options: consolidate options definitions
options: consolidate options definitions

Some config options(interface_driver, use_namespaces) are defined
multiple times in ad-hoc way.  It causes DuplicateOptError exception
when using those module at the same time.  Right now the exception is
avoided in ad-hoc way by each executable.  Those duplicated
definitions should be consolidated and treated in uniformed way.

This is the blocker for blueprint: l3-agent-consolidation

neutron.services.loadbalancer.drivers.haproxy.agent periodic_interval
conflicts with neutron.service one. Since there is no way to fix it
without changing existing behavior/default value, it is untouched for now.

Closes-bug: #1279769
Change-Id: Ifed79b7ee0033644854499416f8a2b22a20416fe

openstack/neutron-vpnaas,citrix-openstack-build/neutron-vpnaas,openstack/neutron-vpnaas,dougwig/x-neutron-vpnaas