Upgrade Log4j2 in Syncope 1.2.X

[Guido Wimmel]

Hi,

I suggest upgrading Logj42 in Syncope 1.2.X from 2.3 to 2.5 (as in 
2.0.0-SNAPSHOT).

2.5 has some nice additional features, in particular 
https://logging.apache.org/log4j/2.x/manual/appenders.html#CustomDeleteOnRollover

If there are no objections, I'd create a JIRA issue (if necessary) and 
do the corresponding change.
I'm only unsure if it's a good idea to commit right now, as there seem 
to be stability problems with the build(?).

Cheers,
  Guido