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 I realized that instead of doing the before_save callback we could use default_value_for here.
```ruby
default_value_for(:priority) { MiqAeDomain.highest_priority + 1 }
```