baremetal: Allow BM_SECOND_MAC to be unset
baremetal: Allow BM_SECOND_MAC to be unset

Currently DevStack registers the second nic of the baremetal
node even if BM_SECOND_MAC is not set or empty. However
an interface with an empty mac address causes dhcp to fail
(bug 1238595). And such operation will get to return a error
after the bug is resolved. So we should not register the
second nic if BM_SECOND_MAC is not set.

Related-Bug: #1238595
Change-Id: Ib3cc77686b72311403ccacbd70ae9cf43e6eb4c9

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