Add retry attempts to github pub key downloads.

[doctoryes]

You might think these retries are un-needed - and so did I until failures actually happened twice this week - such as this one (shown below): http://jenkins.analytics.edx.org/job/load-google-analytics-permissions/295/console

09:53:54 failed: [10.5.0.194] (item={u'github': True, u'type': u'admin', u'name': u'doctoryes'}) => {"changed": false, "dest": "/home/doctoryes/.ssh/authorized_keys", "item": {"github": true, "name": "doctoryes", "type": "admin"}, "msg": "Request failed", "response": "HTTP Error 500: Internal Server Error", "state": "absent", "status_code": 500, "url": "https://github.com/doctoryes.keys"}