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>