YaPI fixes

There is some caching in NetworkInterfaces.ycp, but YaPI has to be stateless. This fix forces rereading the interfaces in each Read/Write call.

Plus there is a fix for Perl warning:

```
Useless use of hash element in void context at
        /usr/share/YaST2/modules/YaPI/NETWORK.pm line 89 (#1)
```