openstack: throttle job scheduling by default

openstack: throttle job scheduling by default

When running a suite with 50 jobs, it will schedule about 100 instance
creation within less than a minute. It is likely to exceed the API
quotas of the OpenStack provider (number of instance creation per
minute) and lead to instance creation failures.

Set the teuthology-suite to be called with --throttle 15 by default,
that is about 60 / 15 = 4 * 2 = ~8 server creation per minute.

http://tracker.ceph.com/issues/12977 Fixes: #12977

Signed-off-by: Loic Dachary <loic@dachary.org>