Allow to override local_setttings.py template

Add the ability to override the location of the template
used to generate local_setttings.py configuration file.

This could be useful in situations where the provided template
isn't flexible enough to allow the configuration of some settings.

The value still defaults to 'horizon/local_settings.py.erb'.

Change-Id: I8dca1324324b5c96487b9e900b1fe89a38943dcd