Fix tempest flavors and DEFAULT_INSTANCE_TYPE
Fix tempest flavors and DEFAULT_INSTANCE_TYPE

The flavor selection was broken if DEFAULT_INSTANCE_TYPE is defined
but not yet created, for example when it is created in local.sh.
This also has the side effect of setting flavor_ref_alt to the first
flavor where it was unset in the previous code.

Change-Id: I1fa48b3f90af45144c92298b6b07a4f7ee3b499f

Yuriy-Leonov/devstack,prithivm/branch01,costingalan/devstack,vishnugonela/devstack,mssumanth/devstack,pacinete/devstack,HPCHub/devstack,nawawi/openstack,group-policy/devstack,upadhyay-prashant/devstack,bq-xiao/devstack,prithivm/branch01,yamt/devstack,virtualopensystems/devstack,parul24/devstack,openstack-dev/devstack,tianweizhang/devstack,mc2014/devstack,neerja28/Devstack_GlusterFS,LoHChina/devstack,pacinete/devstack,bswartz/devstack,ewindisch/raspi-devstack,bswartz/devstack,richliu/devstack,bigswitch/devstack,olivierlemasle/devstack,sstrato/devstack,bljgaurav/openstack-test,dirkmueller/devstack,nawawi/openstack,mc2014/devstack,kplus/devstack-moonshot,Juniper/devstack,eshnil2000/devstack,andrewrothstein/devstack,upadhyay-prashant/devstack,rushiagr/devstack,CloudServer/devstack,shootstar/devstacktest,noironetworks/devstack,NaokiMiyata/devstack,openstack-dev/devstack,avvocatodemarchis/devstack,j4/devstack,flavio-fernandes/devstack,williamthegrey/devstack,liuquansheng47/devstack,varunarya10/devstack,pczerkas/devstack,olivierlemasle/devstack,kplus/devstack-moonshot,HPCHub/devstack,Millnert/contrail-devstack,thomasem/devstack,mmasaki/devstack,jamielennox/devstack,atulpatil301/devstack,nuxeh/devstack,TwinkleChawla/devstack,vishnugonela/devstack,herrsechs/devstack,ewindisch/raspi-devstack,noironetworks/devstack,promptworks/devstack,inkerra/devstack,wenhuizhang/devstack,eharney/devstack,tianweizhang/devstack,sc68cal/devstack,ozamiatin/devstack,innodee/openstackk,thomasem/devstack,vsham20/devstack,vsham20/devstack,wenhuizhang/devstack,dtroyer/devstack,affo/devstack,sc68cal/devstack,sequenceiq/devstack,richliu/devstack,j4/devstack,dirkmueller/devstack,costingalan/devstack,bq-xiao/devstack,group-policy/devstack,flavio-fernandes/devstack,Mahito/devstack,cloudbuilders/devstack,BSJAIN92/OpenStack,bigswitch/devstack,mssumanth/devstack,promptworks/devstack,pombredanne/devstack,atulpatil301/devstack,mmasaki/devstack,ozamiatin/devstack,parul24/devstack,TwinkleChawla/devstack,samgoon/devstack,cloudbuilders/devstack,affo/devstack,LoHChina/devstack,liuquansheng47/devstack,rushiagr/devstack,pombredanne/devstack,herrsechs/devstack,samgoon/devstack,andrewrothstein/devstack,varunarya10/devstack,nuxeh/devstack,armando-migliaccio/devstack,eharney/devstack,neerja28/Devstack_GlusterFS,sequenceiq/devstack,pczerkas/devstack,Mahito/devstack,CiscoSystems/devstack,dtroyer/devstack,bljgaurav/openstack-test,deepakselvaraj/devstack,JioCloud/devstack,BSJAIN92/OpenStack,srics/devstack,avvocatodemarchis/devstack,CloudServer/devstack,NaokiMiyata/devstack