Experimental scenario for setting IPMI credentials

We need a way to enroll a node without known IPMI credentials. The following changes should be done to support it:

On initiating discovery:
* Inspect a node for extra[ipmi_setup_credentials] (boolean, default False)
   NOTE: replaced by HTTP API argument 'setup_ipmi_credentials' in https://blueprints.launchpad.net/ironic-discoverd/+spec/v1-api-reform
* If it's set to true, treat credentials as desired, not existing.
** Don't validate power management interface
** Don't power on the machine, leave it to the user.

On receiving discovery data and finding a node:
* Inspect a node for extra[ipmi_setup_credentials] again
   NOTE: replaced by options persisted in the database in https://blueprints.launchpad.net/ironic-discoverd/+spec/v1-api-reform
* If it's set, return `ipmi_setup_credentials` in `options` to the ramdisk
* Ramdisk uses ipmitool to set the credentials returned as 'ipmi_username', 'ipmi_password'
* Discoverd to wait for the node power interface to pass validation, only afterwards force power off

Note that we still need a node enrolled with a driver and ipmi_address.

UPDATE: this feature is going to have experimental status for 1.0.
SEE bug #1408635 for further work.