Custom gitlab-shell home path and authorized_keys file path

I noticed some small issues.

1. GitLab Shell must be deployed under user home directory, however, the [installation doc](https://github.com/gitlabhq/gitlabhq/blob/master/doc/install/installation.md#4-gitlab-shell) does not specify it with an **Important Note**.
1. `Authorized_keys` file path is customizable in GitLab Shell, but it is hard-coded in GitLab.

So, here comes this pull request, it's convenient especially for development.