(maint) log uid and gid for Puppet::Util::Execution.execute if specified as non-default

This might be able to use a little bit of cleanup, I guess it depends on how nicely you want the output formatted. But I was debugging a custom provider that uses Puppet::Util::Execution.execute and seemed to be executing as root instead of the specified user... and found that the debugging output didn't really help me find the problem.
This could also be shortened to `if options[:uid]`, thoughts?