[schema deserial] fix loading of schema when mapping an entity type to an existing one
To ensure rdef of the old entity type don't overwrite those already existing
on the mapped entity type.
Also rename some variables/arguments on the way to make things clearer.
from os.path import join
from cubicweb.toolsutils import create_dir
option_renamed('pyro-application-id', 'pyro-instance-id')
create_dir(join(config.appdatahome, 'backup'))