Track whether a node is installed or not

We now track the installation state of a node: once an installer finishes
and reports that by doing a GET against stage_done_url('finished'), we
remember the name of the policy and the current timestamp in the node. This
information will stick around until a node is bound to another policy.
Add a 'delete_policy' command

The command will remove the policy from any nodes bound to it, but keep the
nodes marked as 'bound' so that they will boot locally.
Add calls to stage_done_url('finished') to installers missing it

Doing this is now crucial in the installation process; without such a call,
the node will never be marked as 'installed' and therefore won't be
protected when its policy is removed.