Change StopInstances to take []instance.Id

Change StopInstances to take []instance.Id

StopInstances previously took []instance.Instance,
which is unnecessary for most providers, and imposes
an unnecessary cost for callers that only have IDs
initially. We change StopInstances to 

The openstack provider's StopInstances is now slightly
more expensive than before, requiring an additional call
to Instances. This cost will disappear when we fix the
security groups/open-port implementation.

Drive-by: the maas provider now uses a bulk "release"
in StopInstances.

Live tested on ec2, local, virtual MAAS, canonistack, azure.
I lack a Joyent account, but I think it should be fine.

Fixes lp:1260171
Fixes lp:1316272

https://code.launchpad.net/~axwalk/juju-core/lp1260171-stopinstances-ids/+merge/219347

(do not edit description out of merge proposal)

[axw, 2014-05-13 11:58:10]

Please take a look.

[dave_cheney.net, 2014-05-13 21:36:31]

On 2014/05/13 11:58:10, axw wrote:
> Please take a look.

SGTM. How do you feel about making StopInstance variadic ?

[wallyworld, 2014-05-14 02:37:16]

LGTM, agree with Dave's comment so would be good to do that before landing

https://codereview.appspot.com/97400043/diff/1/container/interface.go
File container/interface.go (right):

https://codereview.appspot.com/97400043/diff/1/container/interface.go#newcode31
container/interface.go:31: // Instance.
comment tweak - "by instance id"

[axw, 2014-05-14 02:48:47]

On 2014/05/13 21:36:31, dfc wrote:
> On 2014/05/13 11:58:10, axw wrote:
> > Please take a look.
> 
> SGTM. How do you feel about making StopInstance variadic ?

Will do.

[axw, 2014-05-14 03:05:04]

Please take a look.

https://codereview.appspot.com/97400043/diff/1/container/interface.go
File container/interface.go (right):

https://codereview.appspot.com/97400043/diff/1/container/interface.go#newcode31
container/interface.go:31: // Instance.
On 2014/05/14 02:37:16, wallyworld wrote:
> comment tweak - "by instance id"

Done.

[dave_cheney.net, 2014-05-14 03:31:12]

On Wed, May 14, 2014 at 12:37 PM,  <ian.booth@canonical.com> wrote:
> LGTM, agree with Dave's comment so would be good to do that before
> landing

Making StopInstance variadic would make some callers initially more verbose, ie

var i []instance.Instance
p.StopInstances(i...) // note variadic expansion to pass the contents
of the slice, not the slice itself.

but, as I found with the change that I made to Addresses recently,
most of the callers that are creating those slices are doing so to
pass one item, so they can be refactored to be variadic themselves and
everything becomes a lot simpler to read without so much tedious
boxing of single values into slices.