support headers in template fetch

[chungers]

This will make it possible to use the templates to fetch metadata on GCP: https://cloud.google.com/compute/docs/storing-retrieving-metadata#querying

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

[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 "template-headers" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353799136
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 #444 into master will increase coverage by 0.5%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #444     +/-   ##
=========================================
+ Coverage   69.81%   70.31%   +0.5%     
=========================================
  Files          74       68      -6     
  Lines        4343     3925    -418     
=========================================
- Hits         3032     2760    -272     
+ Misses       1034      925    -109     
+ Partials      277      240     -37

Impacted Files	Coverage Δ
pkg/template/util.go
pkg/template/defaults.go
pkg/template/help.go
pkg/broker/server/sse.go	74.78% <0%> (+1.68%)	⬆️
pkg/manager/manager.go	66.06% <0%> (+2.42%)	⬆️
pkg/broker/client/sse.go	68.6% <0%> (+3.48%)	⬆️
pkg/manager/group_plugin_impl.go	19.14% <0%> (+4.25%)	⬆️

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