Expose a method to determine if a zone can be deleted

This will be used by the UI instead of their current, separate logic

app/models/zone.rb

@@ -241,6 +241,13 @@ def ntp_reload_queue
     servers.each(&:ntp_reload_queue)
   end
 
+  def message_for_illegal_delete
+    return _("cannot delete default zone") if name == "default"
+    return _("cannot delete maintenance zone") if self == miq_region.maintenance_zone
+    return _("zone name '%{name}' is used by a server") % {:name => name} if miq_servers.present?
+    _("zone name '%{name}' is used by a provider") % {:name => name} if ext_management_systems.present?
+  end
+
   protected
 
   def remove_servers_if_podified
@@ -257,8 +264,7 @@ def create_server_if_podified
   end
 
   def check_zone_in_use_on_destroy
-    raise _("cannot delete default zone") if name == "default"
-    raise _("cannot delete maintenance zone") if self == miq_region.maintenance_zone
-    raise _("zone name '%{name}' is used by a server") % {:name => name} unless miq_servers.blank?
+    msg = message_for_illegal_delete
+    raise msg if msg
   end
 end

spec/models/zone_spec.rb

@@ -203,6 +203,11 @@
       described_class.seed
       expect(described_class.maintenance_zone.visible).to eq(false)
     end
+
+    it "cannot be destroyed" do
+      described_class.seed
+      expect { described_class.maintenance_zone.destroy! }.to raise_error(RuntimeError)
+    end
   end
 
   context "validate multi region" do
@@ -226,15 +231,30 @@
   end
 
   it "doesn't create a server for the zone when not podified" do
-    zone = Zone.create!(:name => "my_zone", :description => "some zone")
+    zone = FactoryBot.create(:zone)
     expect(zone.miq_servers.count).to eq(0)
   end
 
-  it "fails to destroy a zone with servers when not podified" do
-    MiqRegion.seed
-    zone = Zone.create!(:name => "my_zone", :description => "some zone")
-    zone.miq_servers.create!(:name => "my_server")
-    expect { zone.destroy! }.to raise_error(RuntimeError)
+  describe "#destroy" do
+    before { MiqRegion.seed }
+
+    it "fails for a zone with servers when not podified" do
+      zone = FactoryBot.create(:zone)
+      zone.miq_servers.create!(:name => "my_server")
+      expect { zone.destroy! }.to raise_error(RuntimeError)
+    end
+
+    it "fails for the default zone" do
+      described_class.seed
+      expect { described_class.default_zone.destroy! }.to raise_error(RuntimeError)
+    end
+
+    it "fails for a zone with a provider" do
+      zone = FactoryBot.create(:zone)
+      FactoryBot.create(:ext_management_system, :zone => zone)
+
+      expect { zone.destroy! }.to raise_error(RuntimeError)
+    end
   end
 
   context "when podified" do
@@ -262,8 +282,6 @@
       end
     end
 
-
-
     it ".destroy deletes the server in the zone" do
       MiqRegion.seed
       zone = Zone.create!(:name => "my_zone", :description => "some zone")