There are currently no ways to control what layers to import using python.
The only (not so elegant) way I see atm to do that would be to:
for all layers to be imported:
import osm
delete unwanted objects
--------------
as an alternative: probably there are ways to export OSM data by layer? I am not fully familiar with the functionality of the OSM api, but I think such a thing is possible. Not via the gui export, but using direct calls to the osm db (URL calls, or via one of the osm apis)
--------------
another alternative: preprocess your osm file with python, or another xml parser, and create different files with only the desired objects in.
