Use iSCSI multipath for libvirt

* Multipath daemon allows to reach block devices
via multiple paths for better resiliency and performance.
Multipathd periodically checks the failed iscsi paths
and maintains a list of valid paths. Libvirt can use more
than one iSCSI path when option volume_use_multipath is set
and when multipathd enabled.

Change-Id: I54629656803c4989f7673e8c69d2a820609b5960
Implements: blueprint nova-libvirt-multipath-iscsi

ansible/roles/nova-cell/templates/nova.conf.d/libvirt.conf.j2

@@ -24,3 +24,6 @@ virt_type = {{ nova_compute_virt_type }}
 {% if nova_libvirt_cpu_mode %}
 cpu_mode = {{ nova_libvirt_cpu_mode }}
 {% endif %}
+{% if enable_multipathd | bool %}
+volume_use_multipath = True
+{% endif %}

releasenotes/notes/nova-libvirt-multipath-iscsi-eb961e44e6fec9ff.yaml

+---
+upgrade:
+  - |
+    Enabling multipathd will now configure Nova to use it.