core: allowed to add network label even if Vm using this network is
running.

~ Network label was checked to be unchanged in method
'onlyPermittedFieldsChanged'. This is extracted out into new method
'allowedNetworkLabelManipulation' which returns true if network label
was not changed, or if former label was not specified (null) and new
one is specified (!=null).

~ moved methods out from non static SyncNetworkParametersBuilder.
These methods can be used outside from SyncNetworkParametersBuilder,
but in that case their owning class have to be instantiated using
CommandContext parameter, which is completely unrelated to these four
methods.

Bug-Url: https://bugzilla.redhat.com/1134009
Change-Id: Ic4e37a36bc8ae1b5614c3c6d218fb13a4e59a021
Signed-off-by: Martin Mucha <mmucha@redhat.com>