Fix Haskell log file naming after virtual cluster changes

Commit 3329f4de changed the Haskell log file from constants to
functions, but introduced a bug: it uses now the daemon name instead
of the correct log file, which means "ganeti-confd.log" instead of
"conf-daemon.log".

In order to fix this, we need to abstract the log file base (in
constants.py) into a separate set of constants, so that we can reuse
it in the Haskell code.

This fixes issue 343.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>