Provide hint about the requirement of full metadata when PyACI is used without generating the metadata

[kprav33n]

PyACI doesn't distribute ACI model metadata. It instead comes with a minimal metadata for it's unit tests. User must generate full metadata by connecting to a running APIC of desired version. This is not very obvious as per some users' feedback. When PyACI is started with the minimal metadata, provide hint on how to generate the full metadata.