fix paster db clean again

User.get() leaves an open transaction when the site user is added to the
pylons context, commit here closes it so it does not lock tables when
running paster db clean
fix paster db clean

defer commit from get_site_user and attach it to pylons context first
[#1656] add doc string about defer_commit to get_site_user
Merge branch 'master' of https://github.com/ckan/ckan into 1656-fix-paster-db-clean
Merge branch '1656-fix-paster-db-clean'