Move netmodels.objectivize to configNetwork

This patch resolves a circular import problem which will be introduced
by making netinfo have object. To use objects defined in netmodels,
we need import netmodels in netinfo. It will cause a circular import:
netcf.ifcfg->netinfo->netmodels->netcf.ifcg. Moving objectivize() from
netmodels to configNetwork can remove the importing of netcf.ifcfg in
netmodels. The following is my proposal of network configuration modules:

netmodels - define all kinds of network objects. Each kind of network ojbect
            should has the functions of:
			  validate parameters
			  configuration/remove
			  fetch info from existing devices
			This module shouldn't depend on any other network modules.
netinfo - keep global network info composed of network objects
configNetwork - implement network configuration API objectivize the API parameters
netcf/ifcfg - configurator based on ifcfg

So I think it makes sense to move objectivize to configNetwork
Keep one ip address in IpConfig

I don't understand why keep multiple ip addresses in one IpConfig
object.
fix IpConfig validation

I got an error of 'classmethod is not callable' when testing the code.
We can't use the class method indirectly because it will return the
descriptor itself instead of calling the __get__() method of the descriptor.
Move testcases for ip address/netmask to netmodlesTests