Validate presence of feed on middleware servers

Hawkular's MiddlewareServer relies on feed not being nil.
Adding a `validates_presence` to be sure.

And fixing the factory to always create a feed - to remove the need to always provide `feed` when mocking to prevent errors from `CGI.unescape`.

app/models/middleware_server.rb

@@ -11,6 +11,8 @@ class MiddlewareServer < ApplicationRecord
   serialize :properties
   acts_as_miq_taggable
 
+  validates :feed, :presence => true
+
   def properties
     super || {}
   end

spec/factories/middleware_servers.rb

@@ -1,6 +1,7 @@
 FactoryGirl.define do
   factory :middleware_server do
     sequence(:name) { |n| "middleware_server_#{seq_padded_for_sorting(n)}" }
+    sequence(:feed) { |n| "feed_#{n}" }
   end
 
   factory :hawkular_middleware_server,