Change SshRunner usage

Currently the SshRunner uses a SimpleConfigReader instance, however this
is not best. We change it to use the cluster name directly (and its
constructor now takes this as parameter, instead of SCR), and its
callers are change to pass the name directly.

As a consequence, we can now remove the initialization of SCR in
gnt-cluster (copyfile and command), and instead we query the master for
the cluster name).

Reviewed-by: imsnah