Support MLFlow tracking server ARN in config.yaml

**Describe the feature you'd like**
MLFlow tracking server arn available in config.yaml file schema

**How would this feature be used? Please describe.**
This would be used like other default settings in the config.yaml

**Additional context**
Admins could/would set this resulting in a better user experience as end users would not have to look it up, similar for how we set default bucket, vpc id, security groups, kms key etc in the config.yaml

Start by locating the config.yaml schema and the existing default bucket, VPC ID, security group, and KMS key settings. Trace how those defaults are read, then add the MLFlow tracking server ARN in the same configuration path; done means administrators can set it in config.yaml and users do not need to look it up.

Written by the indexing model from the issue text.