librenms: Provide a database purge script

The syslog and eventlog tables in librenms grow without a limit. That
can cause problems after an amount of time. Provision a python purging
script that will purge everyday the last month of data for syslog,
eventlog and perftimes tables

Change-Id: Idf622aafd06871abc95997b34164c22967b85268
Merge "librenms: Provide a database purge script" into production