Environment variable parsing support for Config files

[deathbyknowledge]

Description of the change

These changes add support for usage of environment variables in Yaml config files(currently only enabled in the Config module).
PyYaml supports node tagging, so this uses a custom constructor that replaces nodes tagged with !ENV with an environment variable with the same name.

This step will explicitly error if a variable is not set.

Example:

foo: !ENV MY_VAR

The value for foo would be set to the value of the MY_VAR variable.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

N/A

Describe multi-tenancy segmentation

N/A