Update `generate_users` rake task

[patcon]

https://github.com/myplanetdigital/jenkins-inception/blob/develop/Rakefile#L22-L95

Things that need fixing:

• currently uses GITHUB_PASSWORD set in envvar, but should use hub gem authentication like the set_service_hook task does.
• currently just takes to first ssh key from the github user, but should probably load all keys from GitHub into the user's data_bag item. (Will want to provide feedback of what's happening, but should be safe to assume that any public key attached to a github account is safe... there's a bigger issue if this isn't true)

[patcon]

Solved by 9a05d13 and 0feac861!