deploy/ci: Add dummy node

Remove all roles from DEFAULT section in setups.conf. This way each
setup has only the roles/nodes that includes in the corresponding
section. Since currently the Context() class expects to find all
roles in each setup (db, mq, etc.) we have to introduce a dummy node
that eventually should not be used.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>