Fix runtime security volumes creation

[clamoriniere]

What does this PR do?

Bug discovered during the QA of #342.

• Fix defaulting of system-probe if security.runtime is enabled.
• Mount the security-runtime empty dir even if policydir is not set.

Motivation

Additional Notes

Anything else we should know when reviewing?

Describe your test plan

Bug discovered with the following DatadogAgent configuration

apiVersion: datadoghq.com/v1alpha1
kind: DatadogAgent
metadata:
  name: datadog
  namespace: datadog
spec:
  credentials:
    apiKey: REDACTED
    appKey: REDACTED
  agent:
    clusterName: foo
    config:
      kubelet:
        tlsVerify: false
    image:
      name: "gcr.io/datadoghq/agent:latest"
    apm:
      enabled: true
    process:
      enabled: true
      processCollectionEnabled: true
    log:
      enabled: true
    systemProbe:
      bpfDebugEnabled: true
    security:
      runtime:
        enabled: true

[codecov-commenter]

Codecov Report

Merging #362 (725b908) into v0.7 (9655bc5) will increase coverage by 0.01%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##             v0.7     #362      +/-   ##
==========================================
+ Coverage   65.04%   65.06%   +0.01%     
==========================================
  Files          63       63              
  Lines        6901     6907       +6     
==========================================
+ Hits         4489     4494       +5     
- Misses       2093     2094       +1     
  Partials      319      319              

Flag	Coverage Δ
unittests	65.06% <72.72%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
api/v1alpha1/datadogagent_default.go	81.12% <40.00%> (-0.08%)	⬇️
controllers/datadogagent/utils.go	84.30% <100.00%> (+0.03%)	⬆️

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[clamoriniere]

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