[TEST] only reset clients on nightly tests

resetting the clients on each test (in after test) makes the tests running, especially in network mode, much slower, since transport client needs to be created each time when randmized to be used. Also, on OSX, the excessive connections causes bind exceptions eventually which makes running the network tests much harder on it.
Right, sniff mode causes problems, the first attempt with sniff was done on this branch: https://github.com/elasticsearch/elasticsearch/tree/enhancement/test-enable-transport-client-sniff
It allows indeed to disable sniffing for some of the tests, mainly when nodes are shut down, also needed since sniffing only takes into account data nodes.