Fix gnt-cluster “command” and “copyfile”

Since the disabling of forking in the master daemon, the two ssh-based
subcommands were not working anymore. However, there is no need at all
for the commands to be run from the master daemon (permissions to read
the cluster private ssh key notwithstanding), they can be run directly
from the command line utilities.

The patch removes the two opcodes OpRunClusterCommand and
OpClusterCopyFile (and their associated LUs) and changes the code in
‘gnt-cluster’ to query the list of nodes and run directly the SshRunner
over the list. As such, all forking is done from the gnt-cluster script,
and the commands are working again.

Reviewed-by: imsnah