Now running varnishkafka on text varnishes

Mark was concerned about memory usage, but the current
buffering level of 500K messages should be ok.  I
believe the high memory usage by varnishkafka that
had previously been seen was before February when
we had a buffer size of 2M messages.

The average message size in webrequest_mobile is
less than 800 bytes.  500000*800 is less than
400 MB.

Change-Id: I0567cf08ea68b956bf1fe07b879b96d6e257d4d6