Rally should not overwrite pre existing templates by default

[gareth-ellis]

Originally a decision was made that rally should always update existing index templates - the reasoning was that the fields in the index could well be updated regularly. We are now in a position where we have a stable layout for the index templates, thus we should now change to only writing templates if they don't already exist, or if a user specifically says they want them overwritten.

The reasoning behind that is it will allow users to set custom lifecycle policies for moving data to frozen and more.

Suggested implementation:

• Check if the index template exists.
• If it does, and the user has not set a new setting - overwrite_existing_templates to true in the [reporting] section of rally.ini, then do not do anything
• If the template does not exist we should create the templates

Where we update index template: https://github.com/elastic/rally/blob/master/esrally/metrics.py#L902