API Change - Instance plugin to optionally output instance details

[chungers]

This PR adds a new boolean to the Instance plugin's DescribeInstances method. This boolean parameter can be set true by the client to request platform-specific details that represent the current state of the resource. This extra data is carried in the opaque Description.Properties.

The CLI for instance plugin has been updated (the describe verb) to support retrieval of the instance properties and display it in tabular or raw form. When displaying in raw form, either JSON or YAML can be the output format.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "details" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842361855624
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #464 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #464   +/-   ##
=======================================
  Coverage   68.42%   68.42%           
=======================================
  Files          68       68           
  Lines        4067     4067           
=======================================
  Hits         2783     2783           
  Misses       1041     1041           
  Partials      243      243

Impacted Files	Coverage Δ
pkg/plugin/resource/plugin.go	83.42% <100%> (ø)	⬆️
pkg/plugin/group/scaled.go	82.35% <100%> (ø)	⬆️
pkg/rpc/instance/service.go	65.85% <100%> (ø)	⬆️
pkg/plugin/group/testplugin.go	76.19% <100%> (ø)	⬆️
pkg/rpc/instance/client.go	94.28% <100%> (ø)	⬆️

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