Some more fixes for CI

Don't merge it yet, opened to discuss how to make our test suite more reliable.

* -v for nosetests to see easily test cases names
* increase timeout for NTP
* use `read_preference` instead of deprecated `slave_okay` in mongo/tokumx test suite [see this page](http://api.mongodb.org/python/current/api/pymongo/collection.html?highlight=read_preference#pymongo.collection.Collection.slave_okay) (we bundle pymongo==2.6.3)

Still some more work to do on 2 or 3 flaky tests.

- [ ] `tests.test_service_checks.ServiceCheckTestCase` line 218, returning 4 instead of 3 service checks