Enhancement for tuning dmlite (DPM Head Node)

This push release contains two small enhancement that permit to tune DPM. It is based on the following hints:
https://svnweb.cern.ch/trac/lcgdm/wiki/Dpm/Admin/TuningHints
The enhancements are:
* ability to configure NSPOOLSIZE for the MySQL dmlite plugin
* ability to configure the memcache dmlite plugin
By default, this PR does not change any configuration.