Consider restricting Periscope deployment to certain nodes

[peterbom]

If users are trying to diagnose a problem that only affects a specific node, they may want to only deploy Periscope to that node, particularly in a resource constrained cluster.

This was prompted by discussion in #219, which contains a couple of implementation suggestions:

• replicaset with podAffinity
• daemonset with nodeSelector

It's hard to assess demand for such a feature, but it at least seems feasible.

One question is how users would specify the nodes to run on, or whether it should be inferred from other configuration as suggested in a comment.

Right now I feel hesitant to add further environment variables for configuration, see #224 which might make sense to address first.