Run pgproto tests in a cluster once vshard rebalancing is fixed

After the changes introduced in !1481 (merged), all pgproto tests were run in a 3-instance cluster. However, due to vshard rebalancing problems, these tests became flaky. As a temporary measure, we decided to revert the tests back to a 1-instance setup, which stabilizes the test runs for now. Once the vshard issues are resolved (see sbroad#848 (closed)), we should return back to running the tests in a cluster-wide setup, as this provides a more comprehensive tests.