[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.

debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl