fix(certificates): use real DNS names in SAN extension

[dimakr]

After enabling peer verification, the cassandra-stress command is failing SSL handshakes with DB nodes, on GCE and Azure backends.
The issue root cause is because SCT uses hostnames instead of real DNS names for DB nodes on these backends. As a result, the hostnames are included in the SAN extension of nodes' certificates. The mismatch causes peer verification failures, as the hostnames do not align with the actual DNS names.

The change ensures retrieval and use of real DNS names for DB nodes on GCE and Azure backends.

Fixes: #9883 (comment)

Testing

• 🟢 GCE based provision test with c-s command with enabled peer verification
• 🟢 Azure based provision test with c-s command with enabled peer verification
• 🟢 : : AWS based provision test with c-s command with enabled peer verification

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)

[fruch]

LGTM