Add master Makefile. Consolidate settings in default_settings.

The goal of this commit is to start aligning the user interface for the
axle-healthcare-benchmark, with the design of pg_tpch:

 - 'prepare' means taking care of all database initialization, generation and
   loading of data.
 - 'run' means running the queries against the database.
 - there is a single script the user needs to edit for settings.

This commit does not contain an implementation for the run target, which is
left for a future commit.