Template processing fixes

[chungers]

This PR

• Updates the vendor dependency of Sprig to include more functions
• Add -o option for template engine to write output to file
• Add vendor dependency to stow for working with different object store backends.

[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-0504" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354133360
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]

Codecov Report

Merging #523 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   56.69%   56.67%   -0.02%     
==========================================
  Files          57       57              
  Lines        3900     3889      -11     
==========================================
- Hits         2211     2204       -7     
+ Misses       1403     1401       -2     
+ Partials      286      284       -2

Impacted Files	Coverage Δ
examples/instance/terraform/plugin.go	62.32% <0%> (-0.05%)	⬇️

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