feature request - ship custom topics only

[Cryptophobia]

From @darshanime on January 18, 2017 9:56

Currently, when we enable shipping logs to a remote syslog server, all the logs are shipped.

Internally, if the message is from the Workflow Controller or from an application deployed via workflow, it is sent to the logs topic on the local NSQD instance. If from the Workflow Router, to the metrics topic.

Similarly, there should be an environment variable that takes in logs from which apps to ship to syslog.
something like:

SHIP_LOGS_1=controller
SHIP_LOGS_2=my-app-foo
SHIP_LOGS_3=my-app-bar 

Copied from original issue: deis/fluentd#82