Set 3m default interval for Openstack input plugin

The Openstack metrics that telegraf collects are not so frequently
changed (number of instances/networks/services etc). There is no
reason to collect them so often. Increasing of the interval will
decrease load on Openstack APIs form telegraf side.

Change-Id: Iba72373bf91b2060d3c0e1b673132960f0b0f25e
1 file changed