[rails6] Use relative paths for layouts

Fixes the following Deprecation warnings:

  DEPRECATION WARNING: Rendering layouts from an absolute path is deprecated.
  (called from render_pdf_internal_rr at /home/travis/build/ManageIQ/manageiq-ui-classic/app/controllers/application_controller/report_downloads.rb:47)

  DEPRECATION WARNING: Rendering layouts from an absolute path is deprecated.
  (called from print_report at /Users/nicklamuro/code/redhat/manageiq-ui-classic/app/controllers/application_controller/report_downloads.rb:124)

  DEPRECATION WARNING: Rendering layouts from an absolute path is deprecated.
  (called from set_summary_pdf_data at /home/travis/build/ManageIQ/manageiq-ui-classic/app/controllers/application_controller/report_downloads.rb:234)

Caused by the Rails 6.0 Upgrade

app/controllers/application_controller/report_downloads.rb

@@ -45,8 +45,8 @@ def render_pdf_internal(report)
   def render_pdf_internal_rr(report, result)
     @options = report_print_options(report, result) # used by the layouts/print
     render(
-      :template => '/layouts/print/report',
-      :layout   => '/layouts/print',
+      :template => 'layouts/print/report',
+      :layout   => 'layouts/print',
       :locals   => {
         :report => report,
         :data   => result.html_rows.join
@@ -122,8 +122,8 @@ def print_report
 
     @options = report_print_options(result.report, result) # used by the layouts/print
     render(
-      :template => '/layouts/print/report',
-      :layout   => '/layouts/print',
+      :template => 'layouts/print/report',
+      :layout   => 'layouts/print',
       :locals   => {
         :report => result.report,
         :data   => result.html_rows.join
@@ -231,7 +231,7 @@ def set_summary_pdf_data
       end
 
       disable_client_cache
-      render :template => '/layouts/print/textual_summary', :layout => '/layouts/print'
+      render :template => 'layouts/print/textual_summary', :layout => 'layouts/print'
     end
   end
 end