improvement(disrupt_add_remove_dc): add timeout to drop keyspace

[jsmolar]

Fixes: https://github.com/scylladb/scylla-enterprise/issues/4507

This change adds a timeout for the DROP KEYSPACE operation in the disrupt_add_remove_dc finalizer.

The keyspace was successfully dropped after executing session.execute('DROP KEYSPACE IF EXISTS keyspace_new_dc').
However, the CQL command occasionally returned a timeout error, even though the keyspace was actually removed.

The exact cause of this behavior is unclear, but increasing the timeout appears to resolve the issue.
The new timeout value matches the one used in a similar context here:

scylla-cluster-tests/sdcm/nemesis.py

Line 5564 in 4db66ea

session.execute(f"DROP KEYSPACE IF EXISTS {keyspace_name}", timeout=300)

Testing

Is not needed

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)