Cleanup the usages of configWriter in configNetwork

We should keep the interaction with configWriter inside configurator.
So this patch changes to construct new configWriter instantance in
ifcfg's __init__() and use configurator to implement the transaction
management of configuration.

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Change-Id: I19310221ec2c5d7c621c121b77ed87d060676d42
Reviewed-on: http://gerrit.ovirt.org/16343
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>