charts/timescaledb-single: allow to override pod security context (e. g. run as different user)

[joschiwald]

• fixed running timescaledb container with user id other than 1000

What this PR does / why we need it

This PR allows to override the security context of the timescaledb pod. This is useful in restrictive environments where you are only allowed to run your workloads with a specific user.

Which issue this PR fixes

• refs #240
• refs #124
• supersedes #125

Special notes for your reviewer

This change is running successfully in a productive openshift environment with restrictive SCCs.

Checklist

• [x] Chart Version bumped
• [x] CLA signed