Standardize use of templates

[chungers]

This PR aims to standardize on a common wrapper around the golang templates library. This is necessary to provide a common set of functions for all use cases of templating in this project. We expect to leverage templates more heavily to make the plugins more cusomizable without code change.

• Previously the vagrant and swarm plugins all use lots of boilerplate code for template processing, this PR creates a common layer and changed the plugins to follow a consistent pattern of externalizing the templates (e.g. the vagrant file used by the Vagrant plugin and the userdata / init bootscript generated by the swarm flavor) as resources that can be located via a URL:
  • as local files - file:///abs/path/to/the/template
  • as http resources - http://host:port/path/to/template
• A new template package provides common abstractions on using templates, with support for
  • inclusion of other templates (similar to Helm Charts)
  • useful template functions from Sprig
  • see integration_test.go for examples.
• Updated the plugins to use a common drop-in replacement of text/template; updated the main.go in each to support specification of template URL with command-line flags.
• Added tests

[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 "templates" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353402680
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: 50.24%)

Merging #353 into master will decrease coverage by 1.86%

@@             master       #353   diff @@
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  Files            33         38     +5   
  Lines          1753       1942   +189   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           1170       1260    +90   
- Misses          467        548    +81   
- Partials        116        134    +18   

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