Bug 863137 - Ensure users have acknowledged risks associated with running gaiatest

This pull adds a requirement for users to acknowledge the risks associated with running the Gaia Test Runner against a Firefox OS instance.

See [bug 863137](https://bugzilla.mozilla.org/show_bug.cgi?id=863137) for details.

If risks are not acknowledged, the user will see:

    ***** ACKNOWLEDGE RISKS ********************************************************
    These tests are destructive and will remove data from the target Firefox OS
    instance as well as using services that may incur costs! Before you can run
    these tests you must first read through https://developer.mozilla.org/en-
    US/docs/Gaia_Test_Runner#Risks and follow the steps to indicate you have
    acknowledged the risks.
    ********************************************************************************

If risks are acknowledged, the user will see:

    ***** WARNING ******************************************************************
    You are about to run destructive tests against a Firefox OS instance. These
    tests will restore the target to a clean state, meaning any personal data such
    as contacts, messages, photos, videos, music, etc. will be removed. The tests
    may also attempt to initiate outgoing calls, or connect to services such as
    cellular data, wifi, gps, bluetooth, etc.
    ********************************************************************************

    To abort the test run hit Ctrl+C on your keyboard.
    The test run will continue in 30 seconds.