trivial: fix openstack local test
TestInstanceGathering in environs/openstack/local_test.go
failed if run on its own, because it needs a bootstrapped
environment (which other tests provide, when all of them
are run). This fixes the test, so it calls BootstrapOnce,
thus it passes both on its own and with other tests.
R=jameinel
CC=
https://codereview.appspot.com/7303101