Fix RPM dependencies.

[FLOC-969](https://clusterhq.atlassian.net/browse/FLOC-969). FLOC-969
Fixes #934, #945.

This also adds an explicit runtime dependency on `openssh-clients` (we depend on at least `ssh` and `ssh-keygen` currently). It would perhaps be better to depend on `/usr/bin/ssh` and `/user/bin/ssh-keygen`, but the dependency should be going away soon anyway.