tests: cloud: if no existing release, create generic fleet

[rcooke-warwick]

This is to solve the test failures for the cloud suite when there is no existing balena cloud release - causing the app creation to fail. In this case we use a generic-arch fleet instead.

Should reduce some of the friction for first deployment

Change-type: patch

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[rcooke-warwick]

@vipulgupta2048 done