testing: Limit the backends we use for SLF4J

Using KazooTestCase becomes really chatty if you happen to have other
backends than just log4j installed for slf4j. Limiting only to
slf4j-log4j-* ensures that the properties file we provide when
starting zookeeper is enough to turn off the chatter.
Merge pull request #115 from nailor/master

Limit the backends we use for SLF4J