Cherry-pick #17606 to 7.x: Add privileged option so as mb to access data dir in Openshift

[ChrsMark]

Cherry-pick of PR #17606 to 7.x branch. Original message:

What does this PR do?

This PR adds privileged: true in securityContext of Metricbeat Daemonset spec file so as to enable access to data volume added in #17429.

Tested with minishift v1.34.2+83ebaab.

Why is it important?

Metricbeat is not able to start in Openshift without this option:

oc -n kube-system logs -f metricbeat-zsfwt
2020-04-06T10:58:56.615Z        INFO    instance/beat.go:622    Home path: [/usr/share/metricbeat] Config path: [/usr/share/metricbeat] Data path: [/usr/share/metricbeat/data] Logs path: [/usr/share/metricbeat/logs]
2020-04-06T10:58:56.615Z        INFO    instance/beat.go:372    metricbeat stopped.
2020-04-06T10:58:56.616Z        ERROR   instance/beat.go:933    Exiting: Failed to create Beat meta file: open /usr/share/metricbeat/data/meta.json.new: permission denied

Related to #17516.

cc: @exekias @jsoriano @blakerouse

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)