Added poll_frequency to wait_until_element and wait_for_ajax

When waiting for spinner or progress bar it's desired to have
increased poll_frequency rather than a default of 0.5 secs.

This may avoid high CPU usage seen while spinner or progress bar.

Also fixed few content view test-cases.
I think timeout and poll_frequency values is debatable and may be improved further. For now the idea is to have the support in place.