Fix readonly handling

The database `readonly` property expects a boolean, but the olc
provider doesn't take care to parse the existing value into a boolean,
thus leading to issues.

Simply applies the same logic applied to `olcMirrorMode` for
`olcReadOnly`.

lib/puppet/provider/openldap_database/olc.rb

@@ -50,7 +50,7 @@ def self.instances
         when %r{^olcRelay: }
           relay = line.split[1]
         when %r{^olcReadOnly: }i
-          readonly = line.split[1]
+          readonly = line.split[1] == 'TRUE' ? :true : :false
         when %r{^olcSizeLimit: }i
           sizelimit = line.split[1]
         when %r{^olcDbMaxSize: }i

lib/puppet/type/openldap_database.rb

@@ -167,6 +167,8 @@ def should_to_s(_newvalue)
 
   newproperty(:readonly) do
     desc 'Puts the database into read-only mode.'
+    newvalues(:true, :false)
+    defaultto(:false)
   end
 
   newproperty(:sizelimit) do