netconf: Add a base configurator class

This patch add a base configurator class to make the code not related
to specific configurator implementation shared by configurator ifcfg
and iproute2.

It renames ifcfg's member configWriter to configApplier, a more generic
name and also applicable to iproute2.

Change-Id: I6a1291409bf96cd54971791aebe9a9e74af325bf
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/16178
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>