Flaky test: TestNetworkDBCRUDMediumCluster

[thaJeztah]

Seen failing on various occasions; for example, in #2393; https://circleci.com/gh/docker/libnetwork/8871

failures look like:

2019/06/09 10:27:00 Closing DB instances...
--- FAIL: TestNetworkDBCRUDMediumCluster (8.22s)
	Location:	networkdb_test.go:113
	Error:		Network existence verification failed

or

=== RUN   TestNetworkDBCRUDMediumCluster
--- FAIL: TestNetworkDBCRUDMediumCluster (2.00s)
	networkdb_test.go:58: Number of nodes for node2 into the cluster does not match 5 != 4

[thaJeztah]

should be fixed by #2458