ZFSSA iSCSI driver cannot add multple initiators to a group

All initiators defined in zfssa_initiator property would be
added to the group.
Also fixed some typos related to initiators error messages.

Change-Id: Iec6c90702e5aafa153b4a7f1e429974ac450afc0
Closes-Bug: #1369750
Fix race condition in ISCSIConnector _disconnect_volume_multipath_iscsi

This is a similar issue as seen in 
https://bugs.launchpad.net/cinder/+bug/1375382

The list of devices returned by driver.get_all_block_devices() in
_disconnect_volume_multipath_iscsi will potentially contain broken 
symlinks as the SCSI devices have been deleted from calling
self._linuxscsi.remove_multipath_device(device_realpath) right before
_disconnect_volume_multipath_iscsi but the udev rule for the symlink 
may not yet have completed.

Adding in a check to os.path.exists() will ensure that we will not 
consider the broken symlinks as an “in use” device.

Change-Id: I79c9627e9b47127d3765fcec5b7e3bacef179630
Closes-Bug: #1375946
Merge "ZFSSA iSCSI driver cannot add multple initiators to a group"
Merge "Fix race condition in ISCSIConnector _disconnect_volume_multipath_iscsi"