[Nova] Scheduler doesn't consider diskless Compute nodes

MOS 9.2

There are 2 types of Compute nodes in the environment:
1) With HDD
2) Diskless blades

While scheduling a VM, nova-scheduler selects Compute host with HDD even when the VM's flavor specifies do not have disk. Weight scheduler always gives higher score to hosts with disk.

In case when VMs boot from volume, using a flavor with disk=0, there is no need for a host with disk. Actually it should be in favor of a host without disk but there is no consideration of the flavor properties in the weighting process.

There should be filters such that, whenever a VM with disk=0 is booted, a host without disk is weighted first during weighting process.
If the required resources are not available in the weighted host then it should look for another host.

Example of such flavor:
root@cic-1:~# nova flavor-show 96
+----------------------------+---------------------------------------------------------------+
| Property | Value |
+----------------------------+---------------------------------------------------------------+
| OS-FLV-DISABLED:disabled | False |
| OS-FLV-EXT-DATA:ephemeral | 0 |
| disk | 0 |
| extra_specs | {"hw:cpu_policy": "dedicated", "hw:mem_page_size": "1048576"} |
| id | 96 |
| name | BAT-boot-from-volume |
| os-flavor-access:is_public | True |
| ram | 4096 |
| rxtx_factor | 1.0 |
| swap | |
| vcpus | 2 |
+----------------------------+---------------------------------------------------------------+