Allow single alias in swift_storage_policy provider

Regex validation for aliases in swift_storage_policy provider does not
allow single alias value, like for exemple:
  swift::storage::policy { '1':
      policy_name    => '3-Replica-Policy',
      policy_aliases => 'green',
This fixes the accepted pattenr to allow single-value for policy_aliases
parameter.

Also, the whole expression is updated to more strictly validate
the pattern, especially when more than 2 aliases are given.

Change-Id: I62c86097c717d2a74c100386cf20e07976571805

lib/puppet/type/swift_storage_policy.rb

@@ -25,7 +25,7 @@ Puppet::Type.newtype(:swift_storage_policy) do
   newproperty(:aliases) do
     desc 'Storage policy aliases'
     validate do |value|
-      value_match = /([a-zA-Z\d\-]+,\s+)+[a-zA-Z\d\-]+/.match(value)
+      value_match = /^(\s*[a-zA-Z\d\-]+\s*)(,\s*[a-zA-Z\d\-]+\s*)*$/.match(value)
       next unless value_match.nil? || !value_match[0].eql?(value)
       fail('aliases must contain only letters, digits or a dash in a comma separated string')
     end