Added ERB tag support when loading system_pay.yml

[fcarrega]

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"] %>