(PUP-2628) Add members to groups instead of full list for Windows

Previously, if you wanted to use the group resource you needed to specify the
entire list of members as the windows provider ignored the auth_membership
setting and was always the authoritative list. This ensures that the windows
provider respects the auth_membership setting.
The `:boolean => true` causes the type to generate a method named `auth_membership?` which returns `true` or `nil`, e.g. 

<pre>
(rdb:1) group = Puppet::Type.type(:group).new(:name => 'Administrators')
Group[Administrators]
(rdb:1) group.auth_membership?
true
</pre>

And `:parent => Puppet::Parameter::Boolean` is necessary so that puppet properly coerces `True`, `true`, `:true`, `yes`, etc into `true` 

Ideally, just setting the `:parent` would be sufficient, but currently `Puppet::Type` has knowledge about its parameter/property types that it shouldn't.