Support external log4j configuration
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

AkihiroSuda/riemann,fullcontact/riemann,vincentbernat/riemann,alq666/riemann,pradeepchhetri/riemann,vixns/riemann,AkihiroSuda/riemann,forter/riemann,DasAllFolks/riemann,yeller/riemann,mirwan/riemann,patrox/riemann,patrox/riemann,mirwan/riemann,pradeepchhetri/riemann,forter/riemann,yeller/riemann,riemann/riemann,fullcontact/riemann,lispmeister/riemann,eric/riemann,alq666/riemann,fullcontact/riemann,irudyak/riemann,eric/riemann,micrub/riemann,LubyRuffy/riemann,nberger/riemann,aphyr/riemann,jeanpralo/riemann,timbuchwaldt/riemann,bwilber/riemann,algernon/riemann,algernon/riemann,bfritz/riemann,lispmeister/riemann,LubyRuffy/riemann,bowlofstew/riemann,Anvil/riemann,algernon/riemann,patrox/riemann,joerayme/riemann,joerayme/riemann,bfritz/riemann,bg451/riemann,bwilber/riemann,micrub/riemann,topecz/riemann,abailly/riemann,robashton/riemann,timbuchwaldt/riemann,jamtur01/riemann,rhysr/riemann,jamtur01/riemann,rhysr/riemann,lispmeister/riemann,nberger/riemann,rekhajoshm/riemann,riemann/riemann,counsyl/riemann,shokunin/riemann,rekhajoshm/riemann,topecz/riemann,forter/riemann,shokunin/riemann,bowlofstew/riemann,mbuczko/riemann,mbuczko/riemann,topecz/riemann,pharaujo/riemann,pyr/riemann,zamaterian/riemann,bmhatfield/riemann,rhysr/riemann,shokunin/riemann,VideoAmp/riemann-1,irudyak/riemann,pyr/riemann,micrub/riemann,moonranger/riemann,moonranger/riemann,counsyl/riemann,cswaroop/riemann,rekhajoshm/riemann,VideoAmp/riemann-1,bg451/riemann,cswaroop/riemann,pharaujo/riemann,mirwan/riemann,jeanpralo/riemann,zamaterian/riemann,Anvil/riemann,DasAllFolks/riemann,bmhatfield/riemann,vincentbernat/riemann,cswaroop/riemann,robashton/riemann,vixns/riemann,aphyr/riemann,abailly/riemann,yeller/riemann