Add support for macports

[mprahl]

In PR #26, MacOS support was added with homebrew. It'd be nice to also add support for macports as an alternative that can be enabled with a role variable.

[ctipper]

Feel free to ignore me but IMHO homebrew action should be removed. I happen to be using macports, but having experimented with homebrew the task seems to be redundant, you need python3 to run the role, and it is trivial to add openssl11 manually.

[ctipper]

Also a small modification but in virtualenv.yml I suggest the following to aid setting up the role:

- name: use the created virtualenv
  set_fact:
    ansible_python_interpreter: "{{ ler53_account_key_dir }}/ansible-lets-encrypt-virtualenv/bin/python"
  when: not ansible_check_mode
  tags:
  - install

[ctipper]

Final note I couldn't get virtualenv to execute without adding virtualenv_command, I'm using python 3

- name: install pyOpenSSL and boto in a virtualenv (Red Hat/ MacOS)
  pip:
    name: "{{ ler53_item.name }}"
    state: "{{ ler53_item.state | default(omit) }}"
    version: "{{ ler53_item.version | default(omit) }}"
    virtualenv: "{{ ler53_account_key_dir }}/ansible-lets-encrypt-virtualenv"
    # This is required for libselinux-python
    virtualenv_site_packages: yes
    virtualenv_command: /opt/local/bin/python -m venv