Hide rescue interface from validate() output

No API exists today to start or stop "rescue" mode, and no
driver supports it. Therefor, the list of supported driver
interfaces returned by:
  GET /v1/nodes/XXX/validate
should not include "rescue".

Co-Authored-By: David Shrewsbury <shrewsbury.dave@gmail.com>

Change-Id: I2a43409087dfa747fdb6930481e5585324ceeb50
Related-bug: #1294288
Merge "Hide rescue interface from validate() output"