remove unused param from tempest config
remove unused param from tempest config

Remove setting of ssh_user param to boto section.
Because boto uses user name to ssh from compute.ssh_user

Change-Id: Ifd5b99ef35eaf126a3c6e0055837c4741353345e

olivierlemasle/devstack,TwinkleChawla/devstack,openstack-dev/devstack,Mahito/devstack,thomasem/devstack,samgoon/devstack,pczerkas/devstack,dtroyer/devstack,mc2014/devstack,group-policy/devstack,ozamiatin/devstack,parul24/devstack,TwinkleChawla/devstack,samgoon/devstack,richliu/devstack,varunarya10/devstack,pombredanne/devstack,mc2014/devstack,bigswitch/devstack,prithivm/branch01,avvocatodemarchis/devstack,openstack-dev/devstack,nawawi/openstack,atulpatil301/devstack,vsham20/devstack,Mahito/devstack,olivierlemasle/devstack,bigswitch/devstack,atulpatil301/devstack,pczerkas/devstack,richliu/devstack,avvocatodemarchis/devstack,noironetworks/devstack,costingalan/devstack,dtroyer/devstack,dirkmueller/devstack,nawawi/openstack,varunarya10/devstack,bswartz/devstack,pombredanne/devstack,parul24/devstack,thomasem/devstack,group-policy/devstack,noironetworks/devstack,costingalan/devstack,bswartz/devstack,dirkmueller/devstack,mssumanth/devstack,prithivm/branch01,ozamiatin/devstack,vsham20/devstack,mssumanth/devstack