define plugin interface for logging

[ketzacoatl]

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.

[schmichael]

With 0.9 introducing logmon and a plugin subsystem, the infrastructure exists to implement logging plugins as #5490 points out.