Feature: Add explicit firewall whitelisting

[stevepeak]

Services should be prevented from external/internal internet connections, therefore explicitly listing dns locations they need to communicate with.

whitelist:
  - api.slack.com

The above yaml will only allow the service to communicate with api.slack.com. All other internet connections will be blocked.

[wilzbach]

Nice idea, but it probably also needs to be able to express more:

• port (maybe port-ranges and port shorthands like http for 80)
• hostname regex (e.g. (api*.slack.com)
• protocol (HTTP, SMTP, ...)
• package types (TCP, UDP, ...)
• package direction: incoming/outgoing
• IPv4, IPv6 addresses (with subnet groups)

Also, this needs to be applied with the local omg CLI too.