Implement virtual cluster support in Python code

- pathutils: Prepend node-specific prefix path
- RPC: Use virtual paths (see vcluster.py)
- SSH: Pass environment variables, use destination's node directory when
  copying files using scp, use GANETI_HOSTNAME to determine hostname

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>