Ability to specify SYSTEM,PRIORITY,ENABLED during import

https://bugzilla.redhat.com/show_bug.cgi?id=1105113

The rake script and import have been modified to allow for 3 attributes
SYSTEM = true|false
PRIORITY = <number>
ENABLED = true|false

This way the user can override these properties during an import without modifying the attributes in the __domain__.yaml
@Fryguy 
The @single_domain variable is used in 2 places and allows for a user to customize a single domain parameter. I would have to pass it thru multiple functions to get it to the place where it is used. Hence I preferred to use an instance variable