[Feature]: Provide good securityContext by default

[jemag]

Describe the current problem

Currently the problem is twofold:

1. Some securityContext are not configurable by the helm chart.
2. Several default values for securityContext are not as secure as they should be

For example, the deployment for the gpu-operator has no configurable securityContext and, by default, it is left empty.
It is doubtful that this deployment needs full capabilities, privilege escalation, etc. Hence it would make sense to provide a good security posture out of the box.

Describe the solution you'd like

Allow configuring securityContexts of the different pods that can be deployed through this helm chart. (partially discussed here #2070)
Provide good default for the various securityContext (aka stricted possible while allowing the container to fulfill its role), e.g.:

  securityContext:
    allowPrivilegeEscalation: false
    capabilities:
      drop:
      - ALL
    readOnlyRootFilesystem: true
    runAsGroup: 2000
    runAsNonRoot: true
    runAsUser: 2000
    seccompProfile:
      type: RuntimeDefault

What is the added value?

This would improve the default security out of the box for helm chart users. If the containers currently support it, there isn't much downside to improving the default security.

It also helps clarifying to the end-user that these values are officially supported and will not cause any problem with the containers (e.g.: readOnlyRootFilesystem: true could for example cause problems for containers expecting to write to specific directories). By having them already specified, the end-user does not need to do extensive testing to ensure it does not cause any issue.