mgr/ssh: Add SimpleScheduler and streamline arg passing

mgr/ssh: Add SimpleScheduler and streamline arg passing

* adding a mechanism to augment specs.placement.nodes
  even when no nodes are provided via dynamic determination
  from host_pools. (NodeAssignment)

* added SimpleScheduler
  implements most simple placment algorithm possible.
  Choose from a shuffled list of hosts.

* streamlined args passed to the public update_$service
  functions. They all take `spec` now.

Signed-off-by: Joshua Schmid <jschmid@suse.de>