This change fixes the wrong section used by manila::network::standalone
to define the network_plugin_ipvN_enabled parameters.
Previously this usage resulted in duplicate resource declaration, and
made this resource type unusable.
This change also replaces usage of undef by $::os_service_default.
These two are effectively same but $::os_service_default is now
globally used.
Closes-Bug: #1965548
Change-Id: Iedd0805ed815f0507d8fe7b7c91afdb5dfc320be