Correct logic for image cache

Fixes bug 1517

Change-Id: I968b0c79c712e8b845cf285279d9183363410a71
Signed-off-by: beierlm <mark.beierl@canonical.com>

installers/full_install_osm.sh

@@ -452,7 +452,7 @@ function install_juju() {
     echo "Installing juju"
     sudo snap install juju --classic --channel=2.8/stable
     [[ ":$PATH": != *":/snap/bin:"* ]] && PATH="/snap/bin:${PATH}"
-    [ -n "$INSTALL_NOCACHELXDIMAGES" ] && update_juju_images
+    [ -n "$INSTALL_NOCACHELXDIMAGES" ] || update_juju_images
     echo "Finished installation of juju"
     return 0
 }