Pin juju version to 2.8.1 and minor fixes in installer

Change-Id: Icc13bf72b664bad928d90e040f2ce55608dad759
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/docker/MON/Dockerfile b/docker/MON/Dockerfile
index 9505034..22451fc 100644
--- a/docker/MON/Dockerfile
+++ b/docker/MON/Dockerfile
@@ -61,7 +61,7 @@
                           python3-n2vc${N2VC_VERSION}
 
 
-RUN python3 -m pip install -U juju
+RUN python3 -m pip install -U "juju==2.8.1"
 
 COPY scripts/ scripts/