define plugin interface for logging

Per the resolution and discussion in #688, it seems sensible to convert the logging subsystem into a plugin system that would enable operators with the ability to ask nomad to send logs to fluentd, AWS, GLEF, or some other plugin that is not currently supported. It would also make it easier for the community to enhance nomad with these plugins.

This request is primarily borne out of the unfortunate situation where operators are currently forced to either run syslog in their logging stack, or to run a "side-car" container where _every task_ has an instance of the logger (fluentd, logstash, etc), which imposes unnecessary overhead when running many tasks on the same host.

Review the resolution and discussion in #688, then inspect Nomad's current logging subsystem to identify its extension boundary. The work is complete when logging destinations such as Fluentd, AWS, and GELF can be provided through a defined plugin interface without requiring a sidecar per task.

Written by the indexing model from the issue text.