(#13008) Allow scheduled task arguments to be specified

Previously, the arguments property of a scheduled_task resource
could not be specified as it would lead to a TypeError when we passed
the should array value to the task scheduler method that expected a
string.

Previously, scheduled tasks arguments could be specified as either a
string or an array. But if an array was specified it could only contain
a single element.

This commit requires that the arguments be specified as a
space-delimited string, e.g. 'arg1 arg2'. It is not a breaking change,
because the ability to specify arguments never worked.