reduce number of fetches for dynamic commands

[chungers]

This PR includes changes to make the command generation more efficient. Specifically there's no need to do two fetches for each cli playbook script - one for determining the backend and one for actual execution.

Signed-off-by: David Chung david.chung@docker.com

[codecov]

Codecov Report

Merging #549 into master will decrease coverage by 0.17%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
- Coverage   56.87%   56.69%   -0.18%     
==========================================
  Files          57       57              
  Lines        3954     3963       +9     
==========================================
- Hits         2249     2247       -2     
- Misses       1419     1430      +11     
  Partials      286      286

Impacted Files	Coverage Δ
pkg/template/template.go	66.66% <0%> (-4.45%)	⬇️

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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 "work-0517" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353891256
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.