Fix storage driver buffer duration default value.

The cache will now hold atlest the minimum number of stats required by the UI and more
if the buffer duration is longer than the default.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
I don't think we should be doing this. What happens if we have less than 60 samples? We don't want to encode this default in multiple places. We're also going to start going into a model where the UI may request less data and keep it locally.