Support external log4j configuration

Support external log4j configuration. If the property
log4j.configuration is set, do not run `riemann.logging/init`.
`riemann.logging/init` prevents the use of external configuration files
because it removes all configured appenders.

So long as `riemann.logging/init`, it is possible to configure log4j
using a log4j.properties

Personally I prefer configuring log4j using external configuration files.
Configuring logging appenders from the riemann config file limits us to the
appenders supported by riemann already, or requires learning the log4j api.