Wire up placement directives to EnsureAvailability api

When "juju ensure-availability --to machine1, machine2" is run, the placement directives are added to the machine template and so are used when the new instance is created. This is similar to how bootstrap works.

The API added by the previous branch needed to be tweaked to hook things up.

Fixes: https://bugs.launchpad.net/juju-core/+bug/1370332

http://reviews.vapour.ws/r/38/