JobScheduler should leave unchanged ${var} if this cannot be substituted by environment variables

Current Situation

• Currently JobScheduler substitutes
  

  <param name="my_param" value="${my_var}"/>

  to nothing if my_var is not set as environment variable.

Desired Behavior

• This should not happen. If JobScheduler can not substitute, just leave the value. Old behavior should be enabled by option.

Resolution

• Add a backslash to a value that should not be substituted by JobScheduler, e.g.

  <param name="my_param" value="\$abcd"/>

Maintainer Notes

• This behavior will not be changed. Escaping $ characters is a widely used means to prevent substitution.