Allow region setting for ironic-neutron-agent

The "os_region" param is missing in the ironic_neutron_agent.ini.j2
file. Without specifying the region, the neutron service will randomly
pick a region for the ironic-neutron-agent. Therefore, a list of
incorrect agents might be created in the neutron database "agents"
table for nodes from other regions. To list all neutron agents, use
'openstack network agent list'.

Change-Id: Idec265230d0ab63b7559d94690c059608dc2617e
Closes-bug: #1853464