PXE driver to validate the requested image in Glance

This patch adds a check in the PXE driver validate() method to check if
the requested image exist in Glance and if it contains the 'kernel_id'
and 'ramdisk_id' properties set which are needed for the PXE boot.

Closes-Bug: #1324840
Change-Id: I42bf7fcb27286afb3db0fda2a3a09ea49c77496b
Merge "PXE driver to validate the requested image in Glance"