Use instance hash to determine rolling update members

[kaufers]

The group hash is committed at the beginning of the update cycle, it it not guaranteed to reflect that actual instance hash of all members. During an update we cannot rely on the group hash to determine if any instances need to be updated; we need to check the instance hash on each member.

[codecov[bot]]

Codecov Report

Merging #852 into master will increase coverage by <.01%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
+ Coverage   48.58%   48.59%   +<.01%     
==========================================
  Files          89       89              
  Lines        8019     8020       +1     
==========================================
+ Hits         3896     3897       +1     
+ Misses       3768     3767       -1     
- Partials      355      356       +1

Impacted Files	Coverage Δ
pkg/plugin/group/scaler.go	90.6% <100%> (+1.13%)	:arrow_up:
pkg/plugin/group/quorum.go	88.37% <66.66%> (-1.88%)	:arrow_down:

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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 "grp-no-hash" git@github.com:kaufers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354328576
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.