netconf: Add config option for network configurator

After adding iproute2 configurator, we need a config option
to indicate which configurator should be used to perform
network configuration.

Note that net_configurator=iproute2 is not yet fully functional. It
lacks:
- source routing
- ipv6
- Persisting and reporting the BOOTPROTO option

Change-Id: I426a38f229131ce6d32568387be7a809b09ae0c1
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/18210
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>