Fix wrong usage of validate_legacy

The current usage is causing the following error in puppet 8.

validate_legacy(validate_re) expects an Integer value, got String
at ["/.../modules/swift/manifests/ringbuilder/rebalance.pp", 23]:

Change-Id: I60fcea3c1a04456898816b844bf9e555a790d1d4

manifests/ringbuilder/rebalance.pp

@@ -19,8 +19,8 @@ define swift::ringbuilder::rebalance(
     ['^(object(-(\d)+)?|container|account)$']
   )
 
-  if $seed {
-    validate_legacy(Integer, 'validate_re', $seed, ['^\d+$'])
+  if $seed and !($seed =~ Integer) {
+    validate_legacy(Pattern[/^\d+$/], 'validate_re', $seed, ['^\d+$'])
   }
 
   exec { "rebalance_${name}":