slaprunner: some work to allow developping the slaprunner into a slaprunner
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.
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