ENG-1404: Hide region CLI flag for cluster creation

[nmiculinic]

What does this PR do?

Since we currently only support us-east-1, until various things have been covered in https://lightningai.atlassian.net/browse/ENG-1402 and done more testing for different regions, we're hiding this CLI parameter, and only allowing us-east-1 region on the backend. This is temporary hiding until multi-region support is properly tested

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[tchaton]

LGTM !