Fix exclude nodes feature

[clamoriniere]

• Update documentation: how to exclude temporary nodes
• Fix the selectNodes() function that was using the Pod labelSelector to filter Nodes.
• Add some logic in the function that inject the NodeName field affinity to merge the NodeAffinity instead of replacing it.

Test plan:

• deploy the examples/foo-eds_exclude-node.yaml EDS in a test cluster.
• wait to have all the daemonset pod created
• add the label on a node. for example: kubectl label node kind-worker extendeddaemonset.datadoghq.com/exclude=foo --overwrite
• the daemonset pod on the labeled node should be deleted
• remove the labelkubectl label node kind-worker extendeddaemonset.datadoghq.com/exclude- --overwrite, the daemonset pod should be reschedule on the node.

[codecov-commenter]

Codecov Report

Merging #41 into master will increase coverage by 1.97%. The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   34.31%   36.29%   +1.97%     
==========================================
  Files          24       25       +1     
  Lines        1658     1736      +78     
==========================================
+ Hits          569      630      +61     
- Misses       1013     1023      +10     
- Partials       76       83       +7     

Flag	Coverage Δ
#unittests	36.29% <86.66%> (+1.97%)	:arrow_up:

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

Impacted Files	Coverage Δ
.../extendeddaemonset/extendeddaemonset_controller.go	66.66% <ø> (+1.91%)	:arrow_up:
pkg/controller/utils/affinity/affinity.go	59.18% <86.66%> (ø)
.../controller/extendeddaemonsetreplicaset/filters.go	78.21% <0.00%> (+4.78%)	:arrow_up:

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