twill dependency on logback

[Martin Serrano]

Hey Devs,

It seems like the twill project goes through some pain to try to 
insulate itself logging frameworks.  I see use of the slf4j API. 
However, the appmaster code has a dependency on logback via the 
org.apache.twill.internal.logging.Loggings class.   The appmaster will 
not start up without this dependency present.  With the dependency code 
as it is now, there is no way to include the logback jar in the 
generated bundle without it being on the current classpath.  I've 
created a ticket (TWILL-215) to make a missing dependency trigger an 
exception at bundle generation time rather than appmaster execution time.

When the logback jar is on my classpath, my client code picks up logback 
instead of our current logger (log4j).  Is this what is expected?  Is 
there any known workaround?  It seems like there may be a case for 
specifying dependencies of the appmaster that are located outside of the 
current jvm classpath.

Thanks,
Martin