[Backport 2024.1] fix(kms): set proper node regions in multi-dc setups

[mergify]

The AWS-KMS code in DB node python class uses shared dictionary
from the DB cluster class for updating the KMS endpoint region.

It was not a problem when DB nodes setup was serial.
In this case shared object was changed by each node but had proper value in needed time frame.

After implementation of the parallel DB nodes setup (#7383)
we started getting problems that only one state of that shared object was being applied for all nodes.

In single-dc setups everything was correct just because there was no diff among DB node's region names.
But in multi-dc setups values from DB nodes started being applied to each other.

So, fix it by just deep-copying that shared dictionary to avoid updates of a shared object.

Closes: #9025

Testing

• https://argus.scylladb.com/tests/scylla-cluster-tests/42b2d981-559b-4b64-b931-289e9f4ab928

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 ce56367)

Parent PR: #9026

[mergify]

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[vponomaryov]

The fix is not required in the 2024.1 branch.