Add the option to create NetworkPolicy for the Agents

[juliogreff]

What does this PR do?

This adds a NetworkPolicy configuration option for the Datadog Agent, Cluster Agent, and Cluster Checks Runner. Currently, it only controls whether or not a NetworkPolicy should be added.

Motivation

This matches the introduction of the creation of network policies in the helm chart, introduced in this PR and follow ups: DataDog/helm-charts#44

Describe your test plan

Add the following configs to a DatadogAgent, and check that the NetworkPolicy doesn't prevent the agents from working properly:

apiVersion: datadoghq.com/v1alpha1
kind: DatadogAgent
metadata:
  name: datadog
spec:
  agent:
    networkPolicy:
      create: true
  clusterAgent:
    networkPolicy:
      create: true
  clusterChecksRunner:
    networkPolicy:
      create: true

Also check that removing the NetworkPolicy or the corresponding agent removes the network policy as well.

[codecov-io]

Codecov Report

Merging #162 into master will increase coverage by 1.03%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
+ Coverage   59.51%   60.55%   +1.03%     
==========================================
  Files          33       34       +1     
  Lines        4456     4728     +272     
==========================================
+ Hits         2652     2863     +211     
- Misses       1593     1648      +55     
- Partials      211      217       +6     

Flag	Coverage Δ
#unittests	60.55% <76.92%> (+1.03%)	⬆️

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

Impacted Files	Coverage Δ
controllers/datadogagent_controller.go	66.66% <ø> (ø)
controllers/datadogagent/common_networkpolicy.go	41.02% <41.02%> (ø)
controllers/datadogagent/agent.go	55.63% <75.38%> (+5.63%)	⬆️
controllers/datadogagent/clusterchecksrunner.go	76.65% <85.54%> (+2.50%)	⬆️
controllers/datadogagent/clusteragent.go	70.74% <85.88%> (+1.94%)	⬆️
controllers/datadogagent/common_rbac.go	73.86% <100.00%> (ø)

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