Even when using existing server wait for yellow status before proceeding
Even when using existing server wait for yellow status before proceeding

This should solve travis issues where ES is run just before the test
suite.

brunobell/elasticsearch-py,AlexMaskovyak/elasticsearch-py,thomdixon/elasticsearch-py,elastic/elasticsearch-py,Garrett-R/elasticsearch-py,gardsted/elasticsearch-py,brunobell/elasticsearch-py,prinsherbert/elasticsearch-py,liuyi1112/elasticsearch-py,veatch/elasticsearch-py,mjhennig/elasticsearch-py,elastic/elasticsearch-py,chrisseto/elasticsearch-py,kelp404/elasticsearch-py,tailhook/elasticsearch-py,konradkonrad/elasticsearch-py