Default priority for domain, set priority based on ordered domain ids

Ability to set domain properties for the following scenarios

(1) Priority not passed in when creating a new instance, set the priority to be highest priority + 1
(2) Reset the priorities for a ordered array of domain ids, based on the array index
(3) When a domain is deleted squeeze priorities based on (2) 
@mkanoor The common syntax for setting a simple value like this is:
```default_value_for :system, false```