Resolve Elasticsearch templates once at startup

[portante]

Each time the PbenchTemplates() is instantiated (for each Report() object in the cron jobs and once in the indexing code), the on-disk JSON templates are loaded and compared against the database.

We should change this so that there is a module method invoked at pbench-server start time which will perform that work once, update the database, and have the PbenchTemplates() object always pull the templates from the database.