Set RestEasy to use SLF4J.

RestEasy tries to dynamically determine what to use for logging in
its org.jboss.resteasy.logging.Logger class.  It chooses between
commons-logging, log4j, and slf4j by looking for implementation
specific classes on the class loader.  Unfortunately, since it
looks for Log4j first and Log4j is bundled with Tomcat, its
always going to use Log4j even when not appropriate.  Consequently,
we have to specify SLF4J ourselves.