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
diff --git a/lib/tempest b/lib/tempest
index a78cd74..7695054 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -426,7 +426,6 @@
     iniset $TEMPEST_CONFIG boto aki_manifest cirros-${CIRROS_VERSION}-${CIRROS_ARCH}-vmlinuz.manifest.xml
     iniset $TEMPEST_CONFIG boto instance_type "$boto_instance_type"
     iniset $TEMPEST_CONFIG boto http_socket_timeout 30
-    iniset $TEMPEST_CONFIG boto ssh_user ${DEFAULT_INSTANCE_USER:-cirros}
 
     # Orchestration Tests
     if is_service_enabled heat; then