swarm manager update: demote node then leave in flavor.Drain

[chungers]

This PR address #838 by

• Implement in swarm flavor's Drain for managers:
  1. docker node demote the manager node
  2. either remove the node from swarm or leave the swarm: a. docker node rm if the node is not the current node -- this is the case when the control plane (where infrakit is running) is not the same as the manager node being updated. b. docker swarm leave if the node is the same as the node running the code (the self node).
• Make sure in the Group controller, the 'self' node is never destroyed.

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 "rolling-update-managers" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354610248
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 #841 into master will decrease coverage by 0.02%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
- Coverage   48.45%   48.43%   -0.03%     
==========================================
  Files          89       89              
  Lines        8007     8009       +2     
==========================================
- Hits         3880     3879       -1     
- Misses       3770     3773       +3     
  Partials      357      357

Impacted Files	Coverage Δ
pkg/plugin/group/state.go	83.01% <100%> (-0.32%)	:arrow_down:
pkg/plugin/group/rollingupdate.go	93.68% <100%> (+2.37%)	:arrow_up:
pkg/rpc/mux/server.go	42.7% <0%> (-5.21%)	:arrow_down:

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