Plugin activation

[chungers]

This PR implements some of the features required in #284. Specifically

• New packages pkg/launch and pkg/launch/os are added to implement launch a normal os binary via os exec, which is specified in a configuration json (the command and args required to start the process)
• A new verb for infrakit plugin to start a list of named plugins.
• A new config JSON that maps the plugin names to the 'how' in starting up the process
• Added a new version of the tutorial_test to show case how to start the plugins in one CLI invocation.

TODO

• Incorporate plugin activation in the manager so that manager will be able to launch plugins dynamically based on the config input.
• Implement other executors -- such as plugins implemented as Docker containers or engine plugins.

[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 "284-activation" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353934280
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-io]

Current coverage is 64.88% (diff: 65.54%)

Merging #356 into master will increase coverage by 0.15%

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  Files            39         44     +5   
  Lines          1993       2110   +117   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           1290       1369    +79   
- Misses          570        599    +29   
- Partials        133        142     +9   

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