String-based SystemProperty lists should allow for whitespace comma separation

When reading a SystemProperty that is a list, the code will parse a comma separated string. Currently, whitespaces are not allowed, for example:

this is ok: `user1@example.org,user2@example.org`

but this is not: `user1@example.org, user2@example.org`

When manually adding values, many people tend to add a space after a comma. This should be parsed by Openfire.