Add method delete_all_nics to vm.py

[dwettstein]

This PR adds the method delete_all_nics to vm.py. The logic is basically the same as in the delete_nic method, except that instead of deleting just the NIC with given index, it deletes all.

NOTE: I have not added a dedicated test for this method, because of the same reason as above.

PS: I recommend to merge also PR #720, as it contains some bugfixes for delete_nic method.

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[dwettstein]

Hi @rocknes

I have implemented your requested changes. Is there something left to do for this PR?

Thanks.