fix typo in start_task

This code path isn't normally used in Beaker recipes. Luckily pylint
found this for me:

    beah/wires/internals/twtask.py:177: [E0602, start_task] Undefined variable 'factor'

Time to find a brown paper bag, I suppose...

Change-Id: I686cbe61f8d9194ddad93e32dd889711261e0880
Automatic commit of package [beah] release [0.7.2-1].