Do not insist on ifplugd package

bnc#891517
This is even nicely demonstrated if you run the tests on the testing commit (bccc25c2e09cb99777e743fdbb8796862f2cfe6e) which (now) precedes the refactoring commit (a2244f424834da49c422ba91af1f35cc2be91171):

```
Failures:

  1) LanClass#Packages requires wpa_supplicant package when WIRELESS_AUTH_MODE is psk or eap
     Failure/Error: expect(Yast::Lan.Packages).to include "wpa_supplicant"
       #<Yast::NetworkInterfacesClass:0x000000029398b8> received :Locate with unexpected arguments
         expected: (/(psk|eap)/, "WIRELESS_AUTH_MODE")
              got: ("WIRELESS_AUTH_MODE", "eap")
     # ./src/modules/Lan.rb:1128:in `block (2 levels) in Packages'
     # ./src/modules/Lan.rb:1120:in `block in Packages'
     # ./src/modules/Lan.rb:1113:in `Packages'
     # ./test/lan_test.rb:64:in `block (2 levels) in <top (required)>'

Finished in 0.01232 seconds
6 examples, 1 failure
```

Please do run tests *before* and *after* refactoring.