gdash: remove logbase from reqstats.5xx

As surprising as that sounds, reqstats.5xx can be 0 sometimes, in which
case Graphite returns a 500 with a backtrace that ends with:

GraphError: Logarithmic scale specified with a dataset with a minimum
value less than or equal to zero.

Change-Id: I55360737d29f7e01f53b23e35c1f89922b9db70f
Merge "gdash: remove logbase from reqstats.5xx" into production