Supports DEFAULT group opts in TempestPrivateConfig
Supports DEFAULT group opts in TempestPrivateConfig

Options from the DEFAULT group are not accessible via the config
proxy. Adding support for DEFAULT group opts in TempestPrivateConfig to
solve that.

Partially implements bp:test-accounts

Change-Id: Ia80c703080870427bbacc12a239e385d9927631f

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