Add function to load config without generating it

Renames ``load_config`` to ``load_or_generate_config``
Adds a method called ``load_config`` that just loads the
config.

The main synapse.app.homeserver will continue to use
``load_or_generate_config`` to retain backwards compat.
However new worker processes can use ``load_config`` to
load the config avoiding some of the cruft needed to generate
the config.

As the new ``load_config`` method is expected to be used by new
configs it removes support for the legacy commandline overrides
that ``load_or_generate_config`` supports