[c-c create] make post-create step optional (closes #2712041) In this context the "post-create" step refers to the database's creation and initialization. We want to be able to create a new instance but skip the database creation (`db-create` and `db-init` commands) in automatic setup.

version=3
http://download.logilab.org/pub/cubicweb cubicweb-(.*)\.tar\.gz