Add support for "global scripts" for all machines to execute

[EitanBlumin]

• Add support for "global scripts" that all machines are supposed to execute.
• Maybe split the CommandQueue table into commands and an execution log.
• When getting the next command, left join (outer apply?) the two tables to get the last execution time for each command and host.
• A command in the queue could potentially be "Executed Once" or intended for "Multiple Execution". That should be indicated by a bit column.
• A command could be enabled or disabled. For example, a multiple-execution command could be "disabled" to halt its execution at remote machines.