Added ERB tag support when loading system_pay.yml

I modified the YAML file loader to allow ERB tags. The purpose is to allow parameterization through environment variables.

Example for ```vads_site_id``` :
  
    # Environment variable
    SYSTEM_PAY_SITE_ID = "12345678"

    # config/system_pay.yml
    vads_site_id: <%= ENV["SYSTEM_PAY_SITE_ID"] %>