Add persistent memory as standard resource class

This blueprint proposes to add persistent_memory as one of the standard resource class.
Hypervisor such as KVM/QEMU can provide virtual or physical persistent memory devices to a guest VM. Adding Persistent Memory to standard resource class will allow users to specify Persistent Memory requirements for their VM workloads (e.g., data base workload) that require using persistent memory as a high-speed storage device to gain huge performance improvement.
Bare metal workloads can take advantage of Persistent Memory as well.