latest-bosh-stemcell-warden points to a stemcell that doesn't work out of the box

This is not a problem with the script per se, but the defaults don't work. We can either link latest-bosh-stemcell-warden.tgz to the Trusty one or create a new latest-bosh-trusty-stemcell-warden.tgz link. Up for discussion.

Signed-off-by: Chris Hedley <chris@cghsystems.net>