qa_utils: Virtual cluster support for SSH commands

In virtual clusters, all “gnt-*” and “ganeti-*” commands require
environment variables to be set for the virtual cluster root directory
and node name. The “cmd” script generated by “vcluster-setup” is a
wrapper setting those variables.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>