(#8272) Fixup logging in Windows service provider

We want to use self.debug for logging in the provider, so that log messages are
properly associated with the resource, rather than generically coming from
Puppet.

Also fix the self.instances method to not use an unnecessary extra variable
when collecting.

Reviewed-By: Jacob Helwig <jacob@puppetlabs.com>
(cherry picked from commit 38c181d00e87ecc699c6a3e23dd2155f716a6602)