Add Node.instance_info field

This is necessary for instance-level info accessed by drivers.

Related to blueprint add-node-instance-info

Change-Id: I4688676767a80ac6f1f3d571f0df4ea78bc756e1
Drivers may expose a top-level passthru API

This allows drivers to implement a vendor method which will be executed on a
random conductor upon any POST to
/drivers/<driver_name>/vendor_passthru/<method_name>, where <method_name> is
an arbitrary path segment which will be passed to the driver_vendor_passthru
method on the vendor interface.

Implements: blueprint nodeless-vendor-passthru
Change-Id: Ia0866e9ca59c8eba7e52eabeb8c88076c54ebbfc