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
Fix concurrent deletes in virt driver

The _unprovision function used a class member to track the number
of calls to ironic to check the provision state. When more than one
delete is happening concurrently, the deletes will both add to the
same counter. This commit fixes this issue to use a local counter.

Partial-Bug: #1326364

Change-Id: I2552332445fe0e614970ec28eddf7c1ae94e34be
Merge "Fix concurrent deletes in virt driver"
Merge "PXE driver to validate the requested image in Glance"