change method to install Juju

Change-Id: I78ebd597f71e8357e12f55e89bba1ad582e5afe9
Signed-off-by: Gianpietro Lavado <glavado@whitestack.com>
diff --git a/docker/MON/Dockerfile b/docker/MON/Dockerfile
index 72eec87..4b83d8e 100644
--- a/docker/MON/Dockerfile
+++ b/docker/MON/Dockerfile
@@ -40,7 +40,7 @@
     && rm -rf /var/lib/apt/lists/* \
     && pip3 install pip==9.0.3
 
-RUN pip3 install juju 'pyyaml>=5.1.2'
+RUN python3 -m pip install -U juju
 
 ARG REPOSITORY_BASE=http://osm-download.etsi.org/repository/osm/debian
 ARG RELEASE=ReleaseFOUR-daily