Ensure that setting --group "*" matches only apps that have an HAPROXY_GROUP label defined and ignore apps that don't have it defined.

[vishnu2kmohan]

Currently, passing the command line argument --group "*" to Marathon-LB will cause it to advertise all apps on their service ports (provided there are no app and port-specific overrides present in their app definition), regardless of whether they have an HAPROXY_GROUP label present in their app definition.

This is not secure-by-default because there may be instances of apps that are never meant to be advertised because they explicitly did not set an HAPROXY_GROUP label.

Please alter the matching strategy to only advertise apps that have HAPROXY_GROUP set in their app definition.

This issue is related to #402 in that we'd like to enforce an opt-in model whenever possible.

[brndnmtthws]

I've updated the docs in #410.

[vishnu2kmohan]

Thanks!