Increase openstack-exporter timeout to 45 seconds

Even on moderately sized clouds, openstack-exporter can easily take more
than 10 seconds to return, causing Prometheus to fail to scrape data.

Since the default scrape internal is 60 seconds, we can increase the
default timeout to 45 seconds.

Change-Id: Id8dffc425ff057b1e45103eb53734543bca8be80
Closes-Bug: #1976629