ip_address_show should not raise exception

* The FuelWebClient.ip_address_show raised error if 'ip address show'
command returns nothing (return 1). But caller expects string. So
if ip command returns 1 the method should return empty string.
* Add additional parameter to the method to make it more universal.
The parameter is pipe string / commands and it is possible to grep
within the method

Change-Id: Ibf14c54c4439d8403a1058f0991363e18362ca35