stbt.*_motion: global parameters for motion testing are set in stbt.conf

Much like the templatematching functions, the parameters for `detect_motion`
and `wait_for_motion` can now be set in the users STBT_CONFIG_FILE (or else
are read from the system installed config file). This gives users
flexibility, particularly for the `noise_threshold` parameter whose
value should depend on the quality of the video source. For example, an
analogue source will contain more incidental noise than HD video, and
therefore a lower `noise_threshold` value should be used.