logback.xml: Replace layout by encoder

remove deprecated layout entries from logback files.

As described here these entries cause log messages as in the addressed tickets:
http://stackoverflow.com/questions/3257154/how-to-prevent-logback-from-outputting-its-own-status-at-the-start-of-every-log

Ticket: #8434
Acked-by: Paul
Target: master
Require-book: no
Require-notes: no
Patch: http://rb.dcache.org/r/7554/