gnt-cluster command: Add option to prepend node name to output

With this patch a new option (“-M”, like dsh) is added to “gnt-cluster
command” to prepend the node name to all output lines.

$ gnt-cluster command -M uname -a | grep Linux
node18.example.com: Linux node18.example.com [...]
node19.example.com: Linux node19.example.com [...]

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>