Fix a bug in default windows domain name

Domain name default value is set to nil: when the cookbook generates
winsw xml file template domainname is set to "".

Unfortunately winsw ignore the whole  <serviceaccount> part of the
config when domain name is not specified or empty (see https://github.com/kohsuke/winsw/blob/master/ServiceDescriptor.cs#L573)

In the end, when domain is not specified in jenkins slave, user is
ignored and jenkins service will run as administrator..