Policy for terminating leader node during rolling update

[chungers]

This PR provides one implementation to address the leadership handover required by #838

Since there isn't an explicit way to demote a leader in Swarm mode (even though it's possible to demote a manager to worker), this implementation tries to work around this limitation by making sure the leader node is either never destroyed or is the very last one. In the latter case, the leadership will be transferred to another manager node that has been updated. The cluster should then be able to complete the rolling update by provisioning the very last node, the previous leader.

This PR allows the user to specify an option INFRAKIT_GROUP_POLICY_LEADER_SELF_UPDATE to be either last or never. If the policy is set to be last, then the leader node (as determined by the node running the code and its own self LogicalID == the LogicalID of the instance) will always be the last node to be destroyed. If the policy is set to be never, then the node is always considered 'desirable' and therefore left untouched during the rolling update process where 'undesirable' nodes are destroyed and re-provisioned.

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

[codecov[bot]]

Codecov Report

Merging #839 into master will increase coverage by 0.06%. The diff coverage is 93.1%.

@@            Coverage Diff             @@
##           master     #839      +/-   ##
==========================================
+ Coverage   48.24%   48.31%   +0.06%     
==========================================
  Files          89       89              
  Lines        7980     7994      +14     
==========================================
+ Hits         3850     3862      +12     
- Misses       3773     3774       +1     
- Partials      357      358       +1

Impacted Files	Coverage Δ
pkg/plugin/group/scaler.go	89.47% <100%> (+0.06%)	:arrow_up:
pkg/plugin/group/state.go	83.33% <100%> (+2.08%)	:arrow_up:
pkg/plugin/group/quorum.go	90.24% <100%> (+0.12%)	:arrow_up:
pkg/plugin/group/group.go	45.54% <100%> (+0.28%)	:arrow_up:
pkg/plugin/group/testplugin.go	76.47% <100%> (+0.28%)	:arrow_up:
pkg/plugin/group/rollingupdate.go	89.74% <75%> (-2.15%)	:arrow_down:

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