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
If those are boolean columns, you can also do

```ruby
dom.should be_system
dom.should be_enabled
```