Setting PuppetForge remote URL in repo_v3 constructor.

Repo for APIv3 is using PuppetForge as API client. PuppetForge is using https://forgeapi.puppetlabs.com remote URL by default. So, if you set custom forge URL in Puppetfile run librarian-puppet with --no-use-v1-api, it will ignore your forge URL and use official forge.

I have added constructor for repo v3 which sets PuppetForge remote URL.

@rojdek, are you interested in merging this?