Add some facilities to tune the selection of canary nodes

[L3n41c]

This PR adds two features to select canary nodes in a more fine-grained way:

• a node selector to choose the canary nodes: only the nodes matching the selector will be eligible to be used as canary. Ex. of use:

  spec:
  strategy:
    canary:
      nodeSelector:
        matchExpressions:
        - key: canary
          operator: In
          values:
          - "true"

• a node anti-affinity feature: given a set of label keys, the canary nodes will be chosen so that the corresponding label values are different. Ex. of use:

  spec:
  strategy:
    canary:
      nodeAntiAffinityKeys:
      - service

[codecov-commenter]

Codecov Report

Merging #25 into master will increase coverage by 2.45%. The diff coverage is 51.56%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   33.26%   35.72%   +2.45%     
==========================================
  Files          23       25       +2     
  Lines        1521     1716     +195     
==========================================
+ Hits          506      613     +107     
- Misses        946     1024      +78     
- Partials       69       79      +10     

Flag	Coverage Δ
#unittests	35.72% <51.56%> (+2.45%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...onsetreplicaset/ddaemonsetreplicaset_controller.go	51.12% <6.25%> (-2.31%)	:arrow_down:
pkg/controller/utils/labelselector.go	45.16% <45.16%> (ø)
...xtendeddaemonsetreplicaset/scheduler/predicates.go	13.91% <50.00%> (ø)
.../extendeddaemonset/extendeddaemonset_controller.go	70.20% <63.63%> (+4.12%)	:arrow_up:
...er/extendeddaemonsetreplicaset/strategy/unknown.go	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 7db1b06...fb4e852. Read the comment docs.