Fix subnet api test case for non-default cidr
Fix subnet api test case for non-default cidr

This patch removes the hardcoding and calculates the Gateway-ip
based on the configured value.

Change-Id: I186f3997500407ad26472e63b1d5a9b5bcfe1732
Closes-Bug: #1318667

redhat-cip/tempest,nunogt/tempest,jamielennox/tempest,izadorozhna/tempest,rzarzynski/tempest,dkalashnik/tempest,Juraci/tempest,jamielennox/tempest,pczerkas/tempest,Tesora/tesora-tempest,eggmaster/tempest,tudorvio/tempest,alinbalutoiu/tempest,redhat-cip/tempest,Tesora/tesora-tempest,vedujoshi/os_tempest,danielmellado/tempest,eggmaster/tempest,CiscoSystems/tempest,masayukig/tempest,manasi24/tempest,afaheem88/tempest,JioCloud/tempest,vedujoshi/os_tempest,rzarzynski/tempest,pczerkas/tempest,ebagdasa/tempest,ebagdasa/tempest,queria/my-tempest,afaheem88/tempest_neutron,akash1808/tempest,alinbalutoiu/tempest,akash1808/tempest,cisco-openstack/tempest,izadorozhna/tempest,roopali8/tempest,LIS/lis-tempest,nunogt/tempest,hpcloud-mon/tempest,JioCloud/tempest,rakeshmi/tempest,varunarya10/tempest,bigswitch/tempest,openstack/tempest,queria/my-tempest,vedujoshi/tempest,LIS/lis-tempest,afaheem88/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,Lilywei123/tempest,masayukig/tempest,zsoltdudas/lis-tempest,CiscoSystems/tempest,roopali8/tempest,pandeyop/tempest,afaheem88/tempest_neutron,NexusIS/tempest,vmahuli/tempest,xbezdick/tempest,yamt/tempest,NexusIS/tempest,Mirantis/tempest,hayderimran7/tempest,cloudbase/lis-tempest,vedujoshi/tempest,neerja28/Tempest,neerja28/Tempest,Juniper/tempest,vmahuli/tempest,bigswitch/tempest,tudorvio/tempest,zsoltdudas/lis-tempest,manasi24/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,Vaidyanath/tempest,rakeshmi/tempest,jaspreetw/tempest,Vaidyanath/tempest,manasi24/jiocloud-tempest-qatempest,Mirantis/tempest,xbezdick/tempest,jaspreetw/tempest,manasi24/jiocloud-tempest-qatempest,Juraci/tempest,tonyli71/tempest,flyingfish007/tempest,danielmellado/tempest,Juniper/tempest,cisco-openstack/tempest,tonyli71/tempest,sebrandon1/tempest,openstack/tempest,Lilywei123/tempest,yamt/tempest,hpcloud-mon/tempest,dkalashnik/tempest,cloudbase/lis-tempest,sebrandon1/tempest,pandeyop/tempest,flyingfish007/tempest,hayderimran7/tempest,varunarya10/tempest