Add support for Composer Github tokens

[elvetemedve]

The problem

When a project has private Git repositories added to composer.json, then it needs a Github token in order to access and download the code. Currently hem vm up fails because of missing token setup.

The solution

hem config command should ask for entering the Github token (just like it's done with AWS keys) and run composer config -g github-oauth.github.com <oauthtoken> after VM start, but before provision.