mediawiki: use 'udplog' service alias instead of hard-coding fluorine

In production, udplog is a CNAME for fluorine.eqiad.wmnet; in Beta, there is a
'udplog' instance with the relevant role applied. So 'udplog' will point to the
right location in both cases.

Change-Id: Iaa23f0ee8dffebb149ceaf12d0da2f8d96e16585