don't use s390x machines with multiple NICs
We don't have any test that requires s390x to have multiple NICs,
there are only 3 machines in beaker with this settings, let's not block other people that needs these systems from using them.
bkr list-systems --xml-filter='<system><arch op="==" value="s390x"/></system><key_value key="NR_ETH" op=">=" value="2"/>'
ibm-z-500.lab.eng.rdu2.redhat.com
ibm-z-501.lab.eng.rdu2.redhat.com
ibm-z-502.lab.eng.rdu2.redhat.com