haproxy::backend: Always set $_sort_options_alphabetic

Without this there's a possiblity that the variable isn't defined. This
leads to an unreferenced variable call in line 133.

Author: bastelfreak

manifests/backend.pp

@@ -109,10 +109,12 @@
   if $picked_sort_options_alphabetic == false {
     $_sort_options_alphabetic = $picked_sort_options_alphabetic
   } else {
-    if $options.is_a(Hash) and 'option' in $options {
+    if $options =~ Hash and 'option' in $options {
       if ('httpchk' in $options['option']) {
         warning('Overriding the value of $sort_options_alphabetic to "false" due to "httpchk" option defined')
         $_sort_options_alphabetic = false
+      } else {
+        $_sort_options_alphabetic = $picked_sort_options_alphabetic
       }
     } else {
       $_sort_options_alphabetic = $picked_sort_options_alphabetic