nightly: only use read_config to load final, generated config.py

nightly: only use read_config to load final, generated config.py

This needs to use the config loading routine to find Java `keytool`, but
since it doesn't need to fully load the config, isolate that usage in the
function.  Then read_config() is only ever called once, as is it meant to
be used, once the config.py is generated.

Using `from . import common; common.config = foo` will not always work,
due to some oddities to how the `from` imports work. So the full module
has to be imported in order to make sure its always properly set.