rake update:ui - ensure we find all the engines
rake update:ui - ensure we find all the engines

running update:ui from within manageiq-ui-classic fails to find the other rails engines
running it from rails root does make it see them all - wrapping the task to always run from Rails.root

Testing:

with this change:

```diff
--- a/lib/tasks/manageiq/ui_tasks.rake
+++ b/lib/tasks/manageiq/ui_tasks.rake
@@ -129,7 +129,7 @@ def asset_engines
   # we only read assets from app/javascript/, filtering engines based on existence of that dir
   all_engines.select do |_name, path|
     Dir.exist? File.join(path, 'app', 'javascript')
-  end
+  end.tap { |x| puts x.to_json }
 end

 def asset_engines_plugins_hash
```

run `rake update:ui` in ui-classic (before this PR):

```
{"ManageIQ::UI::Classic::Engine":"/home/himdel/manageiq-ui-classic"}
```

run `rake update:ui` in manageiq (and with this PR even ui-classic):

```
{"ManageIQ::GraphQL::Engine":"/home/himdel/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0/bundler/gems/manageiq-graphql-d08c7ff58785","ManageIQ::UI::Classic::Engine":"/home/himdel/manageiq-ui-classic"}
```

ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic