task/ta-npe-load-12 -- report file purge

nparcel/bin/npmts
* updated the _start() method under the the MtsDaemon class to
 purge_files() before reporting

nparcel/conf/npmts.conf
* placeholder for the new file_cache config option

nparcel/mts.py
* new file_cache attribute
* new method purge_files() which purges the oldest report file instances