[Task]: Refine Infrastructure Policy Read

[simonkurtz-MSFT]

Describe the task

This is a small task:

• In infrastructure/afd-apim-pe/create.ipynb and infrastructure/apim-aca/create.ipynb, step 1 reads the BACKEND_XML_POLICY_PATH multiple times before it applies different settings. Just read it once, then do the replacement.
• Verify that the infrastructures build as expected.