slaprunner: uses sshkeys_authority recipe to manage openssh keys, otherwise they may change at any slapos run

Also makes the interface more readable

This makes easier the development of the webrunner, as it can be deployed within a webrunner

Changes include:
  * new parameter "no-ipv4-frontend" to prevent the request of an ipv4 frontend. The webrunner cannot provide one, so let's get rid of failing promises.
  * new paramaters to define the ports of the ssh servers (for runner-importer and runner-exporter). Then no colliding when servers run on the same IP.