@Dups,
I solved the problem. I should have changed "site-default" folder name as "site". In fact, PW is programmed to change "site-default" folder name, update config file and create new folders under site/assets during installation. However, Openshift does not give read&write permissions by default. They need to be defined via scripts in .openshift/action_hooks/ .
for example, this line should be added to .openshift/action_hooks/build : chmod -R 0777 ${OPENSHIFT_REPO_DIR}php/ (777 at least during installation)
However, I couldn't succeed after many trials. Do you have any experience about these scripts?
ps: Although PW seems to be installed and functioning perfectly at first, I start to get errors after starting to install new modules.