Wait for all success after nova boot with poll

Confirmed:
The novaclient uses only one instance to show the progress [1] and
doesn't consider the real number of created instances. AFAIK the REST
API to create instances will always return the first instance and not
a full list of all created instances. Blueprints to change that [2]
didn't get implemented. There was a bug about that too (within the
last 12 months) but I don't find it anymore. IIRC we accept this
behavior, but don't pin me down on this.

However, there is the REST API request parameter "return_reservation_id"
which could maybe used in the python-novaclient to list all instances
matching this reservation_id [3]. And then the progress can be polled
per instance. If this would make the testing more reliable it's worth
a shot IMO.

References:
[1] novaclient; Mitaka; create; poll for one instance: https://github.com/openstack/python-novaclient/blob/b80d8cb6e6cd1e86c7dc3c99c3e7d92641c00097/novaclient/v2/shell.py#L591-L592
[2] https://blueprints.launchpad.net/nova/+spec/return-all-servers-during-multiple-create
[3] nova api-ref; V2.1; create multiple instances: http://developer.openstack.org/api-ref-compute-v2.1.html#os-multiple-create-v2.1

@Prathyusha

Thanks for taking over this bug. Please provide a patch for review within the next 2 weeks. In case you won't work on this anymore, set it back to "Confirmed" and remove yourself as assignee.