Support only the username/password combination when cloning private repos. This way we avoid copying SSH keys to the servers.

The repo username/password can be specified directly in the repo URL.
To avoid this, include `ask_for_git_auth=1` as a parameter to the deploy task:

    $fab -f medusa_rackspace/tasks.py deploy:ask_for_git_auth=1,server_count=4

And the script will ask (once) for the username/password to include
in the Git clone URL (it will override the user/pass already in the URL)