mediawiki::syslog: restore ':omfile' prefix to output channel filter

It's not required by the version of rsyslog we run on Precise, but it is
required by the one we run on Trusty. I also added some (valid) whitespace for
readability. I tested this config on both distros.

Change-Id: I2e5b3668e48fdb1cc41503264b718b8143cfa235
Merge "mediawiki::syslog: restore ':omfile' prefix to output channel filter" into production