Fix connecting unnecessary iSCSI sessions issue

In Icehouse with "iscsi_use_multipath=true", attaching a multipath
iSCSI volume may create unnecessary iSCSI sessions.

The iscsiadm discovery command in connect_volume() returns all of the
targets in the Cinder node, not just the ones related to the multipath
volume which is specified by iqn.  If the storage has many targets,
connecting to all these volumes will also result in many unnecessary
connections.

There are two types of iSCSI multipath devices.  One which shares
the same iqn between multiple portals, and the other which use
different iqns on different portals. connect_volume() needs to
identify the type by checking iscsiadm the output if the iqn is
used by multiple portals.

This patch changes the behavior of attaching volume:

   1. Identify the type by checking the iscsiadm output.
   2. Connect to the correct targets by connect_to_iscsi_portal().

Closes-Bug: #1382440

Change-Id: I488ad0c09bf26a609e27d67b9ef60b65bb45e0ad
Merge "Fix connecting unnecessary iSCSI sessions issue"