Intermitent add_device_yubikey acceptance test failure

[Simon Davy]

This has occured a couple of times in a full run, but passes fine most time, and when run individually

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FAIL: test_add_device_yubikey (sst.runtests.TestAdd_Device_Yubikey)
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Traceback (most recent call last):
  File "/home/wavy/canonical/dev/sso/work/.env/local/lib/python2.7/site-packages/sst/runtests.py", line 284, in test
    exec self.code in context
  File "/home/wavy/canonical/dev/sso/work/identityprovider/tests/acceptance/devices/add_device_yubikey.py", line 24, in <module>
    email = helpers.login_or_register_account(device_cleanup=True)
  File "/home/wavy/canonical/dev/sso/work/identityprovider/tests/acceptance/shared/helpers.py", line 146, in login_or_register_account
    email = register_account()
  File "/home/wavy/canonical/dev/sso/work/identityprovider/tests/acceptance/shared/helpers.py", line 56, in register_account
    write_textfield('recaptcha_response_field', 'ignored')
  File "/home/wavy/canonical/dev/sso/work/.env/local/lib/python2.7/site-packages/sst/actions.py", line 505, in write_textfield
    textfield = assert_textfield(id_or_elem)
  File "/home/wavy/canonical/dev/sso/work/.env/local/lib/python2.7/site-packages/sst/actions.py", line 493, in assert_textfield
    elem = _get_elem(id_or_elem)
  File "/home/wavy/canonical/dev/sso/work/.env/local/lib/python2.7/site-packages/sst/actions.py", line 800, in _get_elem
    _raise(msg)
  File "/home/wavy/canonical/dev/sso/work/.env/local/lib/python2.7/site-packages/sst/actions.py", line 112, in _raise
    raise AssertionError(msg)
AssertionError: Element with id: 'recaptcha_response_field' does not exist