Fix panic when starting vars plugin out of order

[chungers]

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

[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 "fix-vars-panic" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354515912
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[bot]]

Codecov Report

Merging #834 into master will decrease coverage by 1.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #834      +/-   ##
==========================================
- Coverage   48.23%   47.16%   -1.07%     
==========================================
  Files          89       89              
  Lines        7974     7944      -30     
==========================================
- Hits         3846     3747      -99     
- Misses       3771     3836      +65     
- Partials      357      361       +4

Impacted Files	Coverage Δ
pkg/util/exec/exec.go	29.37% <0%> (-24.6%)	:arrow_down:
pkg/rpc/mux/server.go	42.7% <0%> (-5.21%)	:arrow_down:
pkg/provider/terraform/instance/apply.go	66.66% <0%> (-3.08%)	:arrow_down:
pkg/provider/terraform/instance/plugin.go	85.24% <0%> (-1.82%)	:arrow_down:
pkg/provider/aws/plugin/instance/monitor.go	0% <0%> (ø)	:arrow_up:

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