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/stack.sh b/stack.sh
index 31ea2e1..8864a18 100755
--- a/stack.sh
+++ b/stack.sh
@@ -746,6 +746,9 @@
 # Do the ugly hacks for broken packages and distros
 source $TOP_DIR/tools/fixup_stuff.sh
 
+if [[ "$USE_SYSTEMD" == "True" ]]; then
+    pip_install_gr systemd-python
+fi
 
 # Virtual Environment
 # -------------------