[FLINK-25674] Add drop tables to be idempotent in case of retrials and the related test

[echauchot]

This is to avoid the flakiness of the CassandraConnectorITCase#test*Pojo* tests due to retrials that try to recreate an already created (and potentially populated) table. This is the quick version that still uses Junit4 and RetryRule to quickly fix the Cassandra flakiness issue. The version that uses RetryExtension needs a migration of cassandra tests to junit5 and some parent tests are in flink-streaming and mutualized so it would be done in a next PR.

I also added a commit to replace all string literals by constants to avoid existing copy/paste.

R: @tillrohrmann
CC: @zentol

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[echauchot]

Thanks @zentol for quick review/merge. Time to monitor flakiness now 😄