billing: Fix regression in billing flat file format rendering

Addresses a regression in rendering the configurable billing format
strings. A bug prevented these formats from being respected. Note
that the format strings are now preprocessed by the dCache
configuration system, which means that escape sequences are
processed. You may have to update custom format strings (in
$$ sequences have to be replaced by $$$$).

Target: trunk
Request: 2.7
Require-notes: yes
Require-book: yes
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6065/