Fixing create_tempest_networks

The function would fail if router['external_gateway_info'] is
none and tries to cal network_id, throwing this error:
TypeError: 'NoneType' object has no attribute '__getitem__'
This patch will check first if the external_gateway_info is
None before tries to set net_id

Change-Id: I077fae82fc295552b4c3543020d9c784b1f1d457
Merge "Fixing create_tempest_networks" into juno