Add the firefox driver for running selenium tests

[jarshwah]

The jenkins ansible scripts include the provisioning of the firefox driver. It should be a straight forward copy into this setup.

[timgraham]

Here are the steps since the ansible scripts aren't public:

- name: download geckodriver
  become: yes
  unarchive: src=https://github.com/mozilla/geckodriver/releases/download/v{{ geckodriver_version }}/geckodriver-v{{ geckodriver_version }}-linux64.tar.gz dest=/tmp copy=no

- name: copy executable file for geckodriver
  become: yes
  copy: src=/tmp/geckodriver dest=/usr/local/bin/geckodriver mode=0755 remote_src=yes

The firefox tests haven't worked since the geckodriver was introduced though, so there's little point in adding this right now.