Let the ActorSystem find its configuration outside of the bundle, in the OSGi etc directory, following this order:
    etc/bundle-BUNDLE_SYMBOLIC_NAME.conf
    etc/bundle-BUNDLE_ID.conf
    etc/akka.conf
    akka-actor classpath config (using the original  OsgiActorSystemFactory.actorSystemConfig method.)

Configurations fallback from the most to the less specific one.

For the moment, as I haven't found test on ActorSystem Configuration in OSGi, I did not add test for this feature.
    	modified:   akka-osgi/src/main/scala/akka/osgi/OsgiActorSystemFactory.scala
 patriknw comments commited

        * .conf, .properties, .json configuration files
        * string interpolation
        * take place of the legacy actorSystemConfig
rkuhn comments applied: no more extension  
Merge pull request #984 from Crossing-Tech/master

akka-osgi provides configurations files in the etc directory (no need to repackage the bundle for configuration modification)