Add option to OSM installer to skip caching juju lxd images

A new option '--nocachelxdimages' has been added to
'installers/full_install_osm.sh'. If specified, the installer will
skip caching juju lxd images. The option will also skip crontab
configuration to guarantee that the images are not cached daily.

The change also modifies 'ci_stage_3.groovy' to use that option in
OSM pipeline, which will save installation time.

Change-Id: I7bfc4432d9a1c99e39440daaf559db897944bd22
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/installers/install_osm.sh b/installers/install_osm.sh
index 5a4e228..f3798e4 100755
--- a/installers/install_osm.sh
+++ b/installers/install_osm.sh
@@ -55,6 +55,8 @@
     echo -e "     -K:             Specifies the name of the controller to use - The controller must be already bootstrapped"
     echo -e "     -d <docker registry URL> use docker registry URL instead of dockerhub"
     echo -e "     -p <docker proxy URL> set docker proxy URL as part of docker CE configuration"
+    echo -e "     -T <docker tag> specify docker tag for the modules specified with option -m"
+    echo -e "     --nocachelxdimages:  do not cache local lxd images, do not create cronjob for that cache (will save installation time, might affect instantiation time)"
     echo -e "     --nolxd:        do not install and configure LXD, allowing unattended installations (assumes LXD is already installed and confifured)"
     echo -e "     --nodocker:     do not install docker, do not initialize a swarm (assumes docker is already installed and a swarm has been initialized)"
     echo -e "     --nojuju:       do not juju, assumes already installed"