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.
Default poll_frequncy of 0.5 is desired when just waiting for few seconds which is fine for most of the web-ui elements.
But during scenarios like, waiting for in mins. Like when syncing stuff it is better to increase the poll_frequency and make it a higher value, as we already know that time taken is more.

I have updated ui/sync.py with the new poll_frequency values.