tweak memcached limit on beta (89GB -> 15GB)

The memcached instances in the beta cluster have 16GB of memory but are
still allowed to grow up to 89GB.  I am not sure what will happen once
the RAM is filled up, I guess it would swap and eventually send the
instance to death / OOM kill.

I have limited the beta instances to roughly 15GB which should prevents
them from diyng.

bug: 52378
Change-Id: Ifcef51fd4592c02e2d8a3a01a16930f23541554c