Fix non-interactive interface choosing

Fix non-interactive interface choosing

In interactive mode user choose an interface and its number is used
in the code. In function 'configiface' the actual interface is taken
from NETDEVICES by this number.

In non-interactive mode we have an interface name and it was used as
index in NETDEVICES which failed and the 1st interface was always taken.
It does not matter what interface was passed in NET_DEV variable script
tried to configure eth0.

Fix it with getting interface number by its name.