Disk cache mode get set to cache='none' for ephemeral disk on ceph with disk_cachemodes="network=writeback"

Issue

When disk_cachemodes="network=writeback" is set as cache mode, attached cinder volumes get attached with cache=writeback.
This got fixed with BZ1482921 . With the same setting ephemeral disk on ceph storage is set to cache=none.
As a result disk performance is not as good as with cache=writeback.

    <disk type='network' device='disk'>
      <driver name='qemu' type='raw' cache='none' discard='unmap'></driver>
      <auth username='openstack'>
        <secret type='ceph' uuid='e3086acc-dc76-11e7-95f9-525400f7c7a5'></secret>
      </auth>
      <source protocol='rbd' name='vms/83ce71e1-505c-48c4-88e0-178df2ec3d43_disk'>
        <host name='172.18.0.13' port='6789'></host>
        <host name='172.18.0.15' port='6789'></host>
        <host name='172.18.0.21' port='6789'></host>
      </source>
      <backingStore></backingStore>
      <target dev='vda' bus='virtio'></target>
      <alias name='virtio-disk0'></alias>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'></address>
    </disk>