Support Openstack scheduler hints as a constraint

Support Openstack scheduler hints as a constraint

Adds Openstack provider specific 'os-scheduler-hints' pseudo-constraint
that instead of helping determine the flavor to use, is simply passed
through when creating the server. This depends on the SchedulerHints
extension that can then influence where to spin up the underlying vm.

As the api expects a json object of string to object pairs, the format
of the constraint is simply json too, with basic validation.

https://code.launchpad.net/~gz/juju/os_scheduler_hints_1049858/+merge/131642

(do not edit description out of merge proposal)

[gz, 2012-10-27 07:53:25]

Please take a look.

[jimbaker, 2012-10-29 11:51:37]

+1, LGTM, with one minor

https://codereview.appspot.com/6783059/diff/1/juju/providers/openstack/launch.py
File juju/providers/openstack/launch.py (right):

https://codereview.appspot.com/6783059/diff/1/juju/providers/openstack/launch...
juju/providers/openstack/launch.py:95: hints =
self._constraints["os-scheduler-hints"]
Is this key guaranteed to be set? run_server does not assume it is set, but the
constraint is currently registered. However, the comments on that registration
muse that it might not be registered. Given the flow, maybe should just be:

hints = self._constraints.get("os-scheduler-hints")

[gzlist, 2012-10-29 15:27:07]

On 2012/10/29 11:51:37, jimbaker wrote:
> 
> juju/providers/openstack/launch.py:95: hints =
> self._constraints["os-scheduler-hints"]
> Is this key guaranteed to be set? run_server does not assume it is set, but
the
> constraint is currently registered. However, the comments on that registration
> muse that it might not be registered. Given the flow, maybe should just be:
> 
> hints = self._constraints.get("os-scheduler-hints")

This does look a little confusing, but see
machine.constraints.Constraints.__getitem__ - a KeyError is raised only if the
constraint key name does not exist, and the pre-registered default is returned
if no constraint of that name was given.