Avoid picking new connection if we only have one host defined

[benrubson]

Hello,

This pull request avoid picking new connection if we only have one host defined.
This avoids lines of log every ~15 seconds :

2016-02-25 21:59:32,499 tcollector[84137] INFO: Selected connection: my.tsdb.host:443
2016-02-25 21:59:47,994 tcollector[84137] INFO: No more healthy hosts, retry with previously blacklisted
2016-02-25 21:59:47,994 tcollector[84137] INFO: Selected connection: my.tsdb.host:443
2016-02-25 22:00:03,366 tcollector[84137] INFO: No more healthy hosts, retry with previously blacklisted
2016-02-25 22:00:03,366 tcollector[84137] INFO: Selected connection: my.tsdb.host:443
2016-02-25 22:00:17,721 tcollector[84137] INFO: No more healthy hosts, retry with previously blacklisted

Thank you !

Ben