Remove some hardcoded names/paths from backend.py

This patch does the following:
  - add constants.GANETI_RUNAS = "root", which is used to compute
    the homedir (and thus the .ssh directory) instead of hardcoding
    "/root/.ssh" in backend.AddNode and backend.LeaveCluster
  - add constants.SSH_CONFIG_DIR (currently hardcoded to /etc/ssh) that
    is used in backend instead of hardcoding it (preparation for
    selecting that at ./configure time)
  - some more internal cleanup in backend.AddNode

Reviewed-by: imsnah