It seems like this pull request (merged 23. okt 2015) might fix this issue: https://github.com/puppetlabs/puppetlabs-mongodb/pull/194
With this, you can supply admin username and password to server creation like this:
class {'::mongodb::server':
auth => true,
create_admin => true,
admin_username => "admin",
admin_password => "admin"
}
Now puppet should be able to authenticate against mongodb on subsequent runs.