bug: Fix cli script when RAG is not enabled

[charles-marion]

Issue #, if available: #592

Description of changes:

Fixes the following:

• When RAG is disabled, the script should not try to set a default embedding model since there are none. (It was causing the failure in the issue above)
• Updated the wording enableSagemakerModels since it refers to only text generation models at this time.
• Fixed defaultEmbedding check. It should show a warning if a SageMaker embedding model is selected but none is available.
• Fixed BedRock KB always enabled (even when there is none)
• Fixed asking for a Kendra external index when it's disabled.

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