[Backport 6.2] fix(region_name): handle case of region_name doesn't exist

[mergify]

in #9492 some code changed to assume region_name always exists and when not using AWS, it doesn't exist (or it's empty)

switch to retrieving it in a safer way

Ref: #9492

Testing

• provision test

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit 7d51938)

Parent PR: #9698

[mergify]

Cherry-pick of 7d51938 has failed:

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Unmerged paths:
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	both modified:   mgmt_cli_test.py
	both modified:   mgmt_upgrade_test.py
	both modified:   sdcm/cluster.py
	both modified:   sdcm/cluster_k8s/eks.py

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