gitPoller: add option to use OauthToken to pull from https repos

[noc0lour]

I don't know if the GitPoller is the right place to enable polling with oauth tokens or if the change source should already emit the right https:// git clone URL.

Basically we could enable cloning with https://$token@github.com/$owner/$repo.git.
This would obliterate setting up SSH on the workers (which is not super straightforward with docker containers).