beta: define $wmgParserCacheDBs

The $wmgParserCacheDBs variable is not defined on beta, that caused php
to output a warning, for example with the job loop script. End result:
bunch of fatal errors because the job loops can't find a db named
'Warning:'

bug: 57915
Change-Id: I9192d7d6711fe0a8875166bcbabce9bd1055fa42