Fix ansible-galaxy install error empty yaml file.

If we execute "ansible-galaxy install -r requirements.yml" on a valid empty yaml file, the following error occurs:

    File "/bin/ansible-galaxy", line 764, in execute_install
    roles_left = map(ansible.utils.role_yaml_parse, yaml.safe_load(f))
    TypeError: argument 2 to map() must support iteration

Further `roles_done` was never used, I assume this should have been named `roles_left`.

Signed-off-by: René Moser <mail@renemoser.net>