[Enhancement][opensearch] Support disabling securityContexts

[foosinn]

Is your feature request related to a problem? Please describe.

Running Opensearch via Helm charts on Openshift is currently non trivial. Openshift manages UID and GID of each container per default and ensures it's not running as root.

Describe the solution you'd like

An option to disable all the security contexts in the current helm chart.

Describe alternatives you've considered

Overwriting the security context with an empty {} or null is troublesome. It failes when using subcharts and even more with nested-subcharts.

It's possible to use Helms post-renderer to remove them, but that's not really clean.

Additional context

It should be noted that currently also the Docker-Image of opensearch allows the container to run as opensearch user. This can easily be fixed by modifing the image.