config: rename net_persistence

'persistence' is a too-generic name for a configurable controlling the
persistence method of network definitions. 'net_persistence' would be a
more appropriate name. Let us rename this one quickly before anyone
starts to use the current name.

Change-Id: Ief0ce67962f705fcba9ed556d87a871c24f50e61
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22611
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>