restore: CQL session created out of rpc_addresses

[karol-kokoszka]

Fixes #3411

This PR extends docker-compose.yaml used for integration-tests to include additional network.
Scylla's listen_address belongs to second network.
Scylla's rpc_address belongs to public network.

Scylla uses rpc_address to accept CQL connections when listen_address is used for node2node communication.

Issue #3411 showed that restore procedure in scylla-manager doesn't support such a setup.
This PR addresses that by querying ScyllaAPI to get the RPC address when creating CQL session.

Integration-tests are updated to work fine on current docer-compose environment.

db.go from testutils is moved to another package testutils/db to avoid import cycle.

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Please make sure that:

• Code is split to commits that address a single change
• Commit messages are informative
• Commit titles have module prefix
• Commit titles have issue nr. suffix

[Michal-Leszczynski]

I understand that this issue has been found while using restore, but changes introduced in this PR do not change it's code anywhere. So is it really a restore issue?

Shouldn't commit restore: CQL session created out of rpc_addresses have cluster prefix?

[karol-kokoszka]

@Michal-Leszczynski it's not really the restore issue, but restore is affected the most.