fix(helm): mlserver env var parallel workers

[domsolutions]

Motivation

mlserver has deprecated env var MLSERVER_MODEL_PARALLEL_WORKERS to set the number of parallel workers, if this is set, mlserver will ignore and set to default of 1. This was causing customers to have performance issues as the configured parallel workers was not being set.

Summary of changes

• changed to new env var MLSERVER_PARALLEL_WORKERS in helm charts
• allow to be configurable via helm, previously was fixed to 1

Checklist

• Added/updated unit tests
• Added/updated documentation
• Checked for typos in variable names, comments, etc.
• Added licences for new files

Testing

[lc525]

lgtm