Add integration tests for cassandra, bigtable and etcd

[tomwilkie]

[pracucci]

Cassandra has been added in #2275 while BigTable in #2249. Etcd is still missing.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid. Etcd integration tests missing.