Fix NIC env in case seq is 0
Fix NIC env in case seq is 0

Commit bd770b4 introduced a bug where the INTERFACE_INDEX was not
exported in scripts' environmenti if NIC's seq was 0. Fix this.

Signed-off-by: Dimitris Aragiorgis <4514117c4a99256e19437c7e3407c4ecbf98aec0@grnet.gr>

apyrgio/snf-ganeti,apyrgio/snf-ganeti