Stop ipmitool.validate from touching the BMC

Stop the IPMITool driver from calling 'mc guid' in validate().

Validate is currently called synchronously when API requests are sent to
  GET /v1/node/NNN/validate

While work is ongoing to make the API more asynchronous, this presents
a particular issue in that a user can spam this URL and overwhelm the
hardware node's BMC.

Furthermore, validate() is called internally in several places, which is
further contributing to BMC instability as reported in the related
bug 1320513.

Change-Id: I2414d2b07e2ab86c85ca18bc033368ddf43f7f43
Closes-bug: #1314954
Related-bug: #1320513
Merge "Stop ipmitool.validate from touching the BMC"