Support passing variables in config.pbtxt

[riZZZhik]

Is your feature request related to a problem? Please describe.
Variables like max_batch_size, dynamic_batching, etc. need to be changed based on the hardware environment and third-party claims every run
Right now there isn't a convenient way to do so

Describe the solution you'd like
Support environment variables inside config

example config.pbtxt:

...
max_batch_size: ${MAX_BATCH_SIZE} or $$MAX_BATCH_SIZE
...

and/or pass them to tritonserver command:

tritonserver --model-repository models --custom_variables '{"var1": "value1"}'

Describe alternatives you've considered
I wrote shell script to be runned before tritonserver with sed command to replace based on env
It works, but it isn't convenient and poses issues inside Kubernetes

for config in $(find $MODELS_PATH -type f -name 'config.pbtxt'); do
    sed -i -e "s/max_batch_size: [0-9]\{1,\}/max_batch_size: ${max_batch_size}/g" $config
done