Update Nozzle to use reverse log proxy, Update Spinner

[mattysweeps]

This PR updates the nozzle to use the reverse log proxy. The stackdriver-spinner was also updated

Major changes include:

• decrease retry count from 1000 to 20
• stackdriver-agent uses its own credentials file
• 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

This change is 

[craigdbarber]

In the future I suggest splitting out vendoring changes into their own PRs, as otherwise it makes things very hard to read.