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 first three lines can be reduced to:
```ruby
ids = MiqAeDomain.where('priority > 0').order('priority ASC').collect(&:id)
```