Enable multipath for libvirt iSCSI Volume Driver

Implements: blueprint libvirt-volume-multipath-iscsi

nova/virt/libvirt/volume.py:LibvirtISCSIVolumeDriver can now
make use of multipath (behavior defined by use_multipath flag)
runs a scsi inquiry to the storage vendor (iSCSI portal)
asking for more portals, logins to the discovered portals and uses a
multipath device (/dev/mapper/XX) instead of a singlepath device
(/dev/disk/by-path/ip-IP:PORT-iscsi-IQN-lun-X).

This improves IO speed and robustness, since if one iSCSI portal goes
down, there are still others to take care of IO.

Change-Id: I30489234b8329f576cf2cbb0ef390670dbee5b95
Merge "Enable multipath for libvirt iSCSI Volume Driver"