Fix synapse.config module "read" command (#11145)

`synapse.config.__main__` has the possibility to read a config item. This can be used to conveniently also validate the config is valid before trying to start Synapse.

 The "read" command broke in https://github.com/matrix-org/synapse/pull/10916

 as it now requires passing in "server.server_name" for example.

 Also made the read command optional so one can just call this with just the confirm file reference and get a "Config parses OK" if things are ok.

Signed-off-by: Jason Robinson <jasonr@matrix.org>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>