Use Reverse Log Proxy, Update CI pipeline

[mattysweeps]

This PR updates the nozzle to use the reverse log proxy. Along the way, various code quality changes were made, including lint fixes and a new CI pipeline.

Major changes include:

• decrease retry count from 1000 to 20
• stackdriver-agent uses its own credentials file
• new CI pipline which uses BBL to create a BOSH director for testing
• create project Makefile for performing CI tasks (room for improvement)
• vendor everything with dep and update dependencies
• update to xenial stemcells
• require CF 2.3.0
• move nozzle's cmd binaries into their own packages
• added missing license headers

Things to test:

• [ ] Stackdriver nozzle's BOSH release
  • [ ] fluentD job
  • [ ] stackdriver-agent job (needs updating to latest dependencies)
  • [x] stackdriver-nozzle job
• [x] Stackdriver nozzle's Tile
• [x] Stackdriver spinner CF app
• [ ] Stackdriver-nozzle cmd binaries
  • [x] clear-metrics-descriptors
  • [x] firehose_rate
  • [ ] firehose-to-stdout (binary seems to work, but all my environments are not configured correctly)
  • [x] metrics_stats
  • [ ] repro-metrics-rpc-errors (this sets invalid metric descriptors, might be worth fixing later)

Things I have yet to test:

• BOSH lite manifests (it seems BOSH lite is deprecated, so I didn't bother with these)

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This change is 

[mattysweeps]

Closing this in favor of #224