Only invoke flavor "Drain" on self during rolling update

[kaufers]

Instead of using a complex policy that defines how the current instance should be handled during a rolling update, this simplifies the logic so that the following occurs if Options.Self is configured:

• The Self instance is sorted last (so that it is the last node to have the rolling update applied)
• Flavor Drain is invoked
• Instance Destroy is never invoked

This allows the newly elected leader to resume the rolling update, destroying the "old" leader instance and provisioning a replacement.

Note that this only affects the rolling update flow; in the group deletion flow the "self" instance will be Drain'd and Destroy'd last.

[codecov[bot]]

Codecov Report

Merging #869 into master will increase coverage by 0.04%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #869      +/-   ##
==========================================
+ Coverage   48.78%   48.83%   +0.04%     
==========================================
  Files          91       91              
  Lines        8211     8213       +2     
==========================================
+ Hits         4006     4011       +5     
+ Misses       3835     3833       -2     
+ Partials      370      369       -1

Impacted Files	Coverage Δ
pkg/controller/group/state.go	83.01% <100%> (ø)	:arrow_up:
pkg/controller/group/testplugin.go	77.77% <100%> (+0.76%)	:arrow_up:
pkg/controller/group/group.go	51.96% <100%> (+1.7%)	:arrow_up:
pkg/controller/group/rollingupdate.go	94.82% <100%> (-0.53%)	:arrow_down:
pkg/controller/group/scaled.go	78.84% <42.85%> (-0.96%)	:arrow_down:
pkg/rpc/mux/server.go	47.91% <0%> (+5.2%)	:arrow_up:

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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 "group-self-destroy-drain-only" git@github.com:kaufers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354052464
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.