Nova supports multiple multipath tools,such as zte superpath, multipath etc

when attach the volume to a virtual machine,we can use multipath device.Currently only support "multipath" tool to manage iSCSI volume.
Different storage equipment manufacturers will generally provide matching of multipath tools, such as EMC powerpath, Fujitsu ETERNUS Multipath Driver, zte superpath etc, in order to realize the personalized features.
This blueprint is designed to support multiple multipath tools, adopts the factory class, base class defines a method, each multipath tool to create a subclass, subclass implementation concrete method. Every increase a multipath tool, a new subclass, has the very good extensibility.
Increase iscsi_used_multipath_tool configuration items in nova.conf, according to the configuration to choose different multipath tools.Late early support multipath and superpath, according to the need to expand.