Renaming udp_stats in ganglia.

These stats were originally configured with a slope of "both",
which was incorrect.  I haven't been able to figure out how to
make Ganglia pick up the change to "positive".  Instead, I am
renaming these metrics by prefixing UDP_ to them, which makes
the names less ambiguous anyway.

Change-Id: I188f233122022c6dee94271813c6f56c2cead91a