Refactor get_ems_folders to create less strings

Creating some strings in this method is inevitable, since we are
generating string paths based on this compiled info, but there were a
lot of excessive strings being created because of it:

- Both of the constants in this method were being created every time
  this method was called.  For the DATACENTER_FOLDER_TYPE one, it was
  just used as a comparison as well
- Using << where possible will avoid generating new substrings, so we
  are only creating one new string at most.
- Deferred the `.dup` of full_path until a new string was known that it
  would be generated.  This did require creating a new variable to hold
  on to the existing variable, but that wasn't a big deal and didn't
  increase the number of allocations.

Split up some ternary operators as well just to make running profilers
on this code easier to determine what as causing allocations
(granularity in tracing object allocations in ruby is only down to a
line by line basis).

app/models/miq_request_workflow.rb

@@ -855,20 +855,28 @@ def ems_has_clusters?
     false
   end
 
+  FOLDER_PATH_SEPARATOR  = " / ".freeze
+  DATACENTER_FOLDER_TYPE = "Datacenter".freeze
   def get_ems_folders(folder, dh = {}, full_path = "")
+    path = full_path
     if folder.evm_object_class == :EmsFolder
       if folder.hidden
         return dh if folder.name != 'vm'
       else
-        full_path += full_path.blank? ? folder.name.to_s : " / #{folder.name}"
-        dh[folder.id] = full_path unless folder.type == "Datacenter"
+        path = full_path.dup
+        if path.blank?
+          path << folder.name.to_s
+        else
+          path << FOLDER_PATH_SEPARATOR << folder.name
+        end
+        dh[folder.id] = path unless folder.type == DATACENTER_FOLDER_TYPE
       end
     end
 
     # Process child folders
     @_get_ems_folders_prefix ||= _log.prefix
     node = load_ems_node(folder, @_get_ems_folders_prefix)
-    node.children.each { |child| get_ems_folders(child.attributes[:object], dh, full_path) } unless node.nil?
+    node.children.each { |child| get_ems_folders(child.attributes[:object], dh, path) } unless node.nil?
 
     dh
   end