Image instance plugin + packetnet instance plugin

[chungers]

In this PR:

• Image instance plugin --> create an OS image instance; handles pushing to different backends.
• Packet instance plugin --> bare-metal cloud service Packet.net instance plugin.

Both are for LinuxKit integration.

[codecov]

Codecov Report

Merging #531 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #531   +/-   ##
=======================================
  Coverage   56.66%   56.66%           
=======================================
  Files          57       57           
  Lines        3937     3937           
=======================================
  Hits         2231     2231           
  Misses       1420     1420           
  Partials      286      286

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[GordonTheTurtle]

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

$ git clone -b "imager" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353583680
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.