another fix to mixture of string or array in attribute (FIXME, fix this after 1.3)
fix available list of IDS

this fix issue when you have e.g. vlan0 and vlan1 and vlan0 is deleted,
then old version allow to use id from 1 to 9, but fixed version allows
0 and from 2 to 9.
use array difference as suggeted by lslezak
simplify code as suggested by lslezak
simplify code as suggested by lslezak
Merge pull request #51 from webyast/another_network_data_fix

Another network data fix