Check that the number of fixed_ips is at least one

In DevStack-Gate, we are attempting to run dual stacked, so even when
running IPv6 tests, an IPv4 address will be present.

Change-Id: I303d15cdf9fb458df49e4b8eaf3adc41226bf176
Required-By: I3d416275f77913769b98e77f7e47bed17fc4d1cc
Merge "Check that the number of fixed_ips is at least one"