Exclude loggers implementations from maven releases

[porunov]

Currently janusgraph jars have logger implementations as dependencies. I think this isn't good because in case janusgraph is used in embedded mode those loggers will be populated to the classpath and users will need to exclude them by their own.
I think, ideally, we shouldn't enforce usage of any loggers. Probably we should use only SLF4J interface.
That said, I guess we need to include some logger implementation only in janusgraph distributions and not in janusgraph jars deployed to maven nexus.
I think users should be able to decide which loggers they want to use.