Fix comment conf file - s/space/comma/

Commit fixes comment in 'src/opentsdb.conf' which says list of ZK hosts is a
space separated list, but in reality it's a comma separated list.

See: http://opentsdb.net/docs/build/html/user_guide/configuration.html

Also, it would be great to fix the following at the link I've mentioned - ``192.168.1.1:2181, 192.168.1.2:2181'' << there is no white space, so don't put one there! It's rather confusing. But I don't know where the source of this documentation is.

Thanks.