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)
Merge pull request #187 from vishh/influxdb_optimze

Fix storage driver buffer duration default value.