make USE_SCREEN=False imply USE_SYSTEMD=True

In order to start making the transition in the gate make
USE_SCREEN=False also mean USE_SYSTEMD=True. We'll never actually
declare USE_SYSTEMD=True in the gate (as that doesn't exist for stable
branches), but this will let us roll over the existing transition.

We also have to install systemd-python 234 because we are recording
exception info in the journal, and all versions before that had a bug
in processing that.

Remove the somewhat pointless screen following journalctl commands. We
really don't want or need those, and they tend to build up over time.

Depends-On: I24513f5cbac2c34cf0130bf812ff2df6ad76657c

Change-Id: I6af6d1857effaf662a9d72bd394864934eacbe70
diff --git a/functions-common b/functions-common
index 35b4860..65c38a5 100644
--- a/functions-common
+++ b/functions-common
@@ -1495,22 +1495,6 @@
 
     $SYSTEMCTL enable $systemd_service
     $SYSTEMCTL start $systemd_service
-    _journal_log $service $systemd_service
-}
-
-function _journal_log {
-    local service=$1
-    local unit=$2
-    local logfile="${service}.log.${CURRENT_LOG_TIME}"
-    local real_logfile="${LOGDIR}/${logfile}"
-    if [[ -n ${LOGDIR} ]]; then
-        $JOURNALCTL_F $2 > "$real_logfile" &
-        bash -c "cd '$LOGDIR' && ln -sf '$logfile' ${service}.log"
-        if [[ -n ${SCREEN_LOGDIR} ]]; then
-            # Drop the backward-compat symlink
-            ln -sf "$real_logfile" ${SCREEN_LOGDIR}/screen-${service}.log
-        fi
-    fi
 }
 
 # Helper to remove the ``*.failure`` files under ``$SERVICE_DIR/$SCREEN_NAME``.
@@ -1700,8 +1684,10 @@
             # Only do this for units which appear enabled, this also
             # catches units that don't really exist for cases like
             # keystone without a failure.
-            $SYSTEMCTL stop devstack@$service.service
-            $SYSTEMCTL disable devstack@$service.service
+            if $SYSTEMCTL is-enabled devstack@$service.service; then
+                $SYSTEMCTL stop devstack@$service.service
+                $SYSTEMCTL disable devstack@$service.service
+            fi
         fi
 
         if [[ -r $SERVICE_DIR/$SCREEN_NAME/$service.pid ]]; then