veil-env-helper: support optional secrets, remove comma-separated list
veil-env-helper: support optional secrets, remove comma-separated list

There's not too many call sites, so we've made life a little easier for
us by stopping to support comma-separated lists. Multiple `-m` and `-s`
are the way to go.

Signed-off-by: Stephan Renatus <3d1a7d0a294b6d8e799ad6e122143905f17f9530@chef.io>

chef/chef_secrets,chef/chef_secrets