Fixes bug 844160
Makes the servers create API call work with all schedulers, removes
'zone boot', and folds create_instance_helper back into servers
controller.
Notable changes:
1) compute API's create_at_all_once has been removed. It was only used
by zone boot.
2) compute API's create() no longer creates Instance DB entries. The
schedulers now do this. This makes sense, as only the schedulers will
know where the instances will be placed. They could be placed locally or
in a child zone. However, this comes at a cost. compute_api.create() now
does a 'call' to the scheduler instead of a 'cast' in most cases (* see
below). This is so it can receive the instance ID(s) that were created
back from the scheduler. Ultimately, we probably need to figure out a
way to generate UUIDs before scheduling and return only the information
we know about an instance before it is actually scheduled and created.
We could then revert this back to a cast. (Or maybe we always return a
reservation ID instead of an instance.)
3) scheduler* calls do not return a host now. They return a value
that'll be returned if the caller does an rpc.call(). The casts to
hosts are now done by the scheduler drivers themselves.
4) There's been an undocumented feature in the OS API to allow multiple
instances to be built. I've kept it.
5) If compute_api.create() is creating multiple instances, only a single
call is made to the scheduler, vs the old way of sending many casts. All
schedulers now check how many instances have been requested.
6) I've added an undocumented option 'return_reservation_id' when
building. If set to True, only a reservation ID is returned to the API
caller, not the instance. This essentially gives you the old 'nova
zone-boot' functionality.
7) It was requested I create a stub for a zones extension, so you'll see
the empty extension in here. We'll move some code to it later.
8) Fixes an unrelated bug that merged into trunk recently where zones DB
calls were not being done with admin context always, anymore.
9) Scheduler calls were always done with admin context when they should
elevate only when needed.
10) Moved stub_network flag so individual tests can run again.
* Case #6 above doesn't wait for the scheduler response with instance
IDs. It does a 'cast' instead.
Change-Id: Ic040780a2e86d7330e225f14056dadbaa9fb3c7e
0fab78825e