Multiplex flavor plugins by type in a single RPC endpoint.

[chungers]

Similar to #360 this PR adds support for sub-types in a flavor plugin.

• Same naming mechanism: plugin.Name := $lookup[/$type] where $type is defined by the name a flavor SPI implementation is bound in the function to export the object in the RPC server (the PluginServerWithTypes method.
• Added tests
• Updated the swarm flavor plugin as two spi flavor.Plugin implementations: one as manager and one as worker -- so the Plugin names should be flavor-swarm/manager and flavor-swarm/worker.
• Swarm flavor plugin no longer needs to implement a Type switch in its own Spec schema. Instead, the switch is handled at the RPC layer.
• Added a new e2e test case, scripts/e2e-swarm.sh and updated the scripts/e2e-test-plugins.json template to describe how to start up the vagrant and swarm flavor 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 "multiplex-flavor" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354446536
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 64.82% (diff: 50.76%)

Merging #364 into master will decrease coverage by 0.56%

@@             master       #364   diff @@
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  Files            45         47     +2   
  Lines          2190       2266    +76   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           1432       1469    +37   
- Misses          611        637    +26   
- Partials        147        160    +13   

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