Hyper-V: Skip logging out in-use targets

As most of the Cinder volume drivers that use iSCSI export one LUN
per iSCSI target, the Hyper-V driver was logging out the according
target each time a volume was disconnected.

This causes issues with some volume drivers that use multiple LUNs
per iSCSI target and may disconnect in-use targets.

This patch fixes the issue by checking if there are other LUNs
being used, disconnecting the iSCSI targets otherwise.

Change-Id: Ida08e7129a7b7940ddea488488cccc7571a43b76
Closes-Bug: #1372815
VMware: trivial fix for comment

The patch makes a few minor changes to a comment that was added in
commit I7598afbf0dc3c527471af34224003d28e64daaff

TrivialFix

Change-Id: Ic12b0b17aac6f745e62bade37204af516e50df0d
Use six.text_type instead of unicode

According to 6683905b532b6030989064636aa5506d4cbe5b22 ,
we shouldn't use unicode in an exception message.

Change-Id: I377f0cf7b8a66026491b75e89b470a328670f17a
Updated from global requirements

Change-Id: I0f3cb3023952f889650c3ebbd0fb6aee5861898d
Merge "VMware: trivial fix for comment"
Merge "Use six.text_type instead of unicode"
Merge "Hyper-V: Skip logging out in-use targets"
Merge "Updated from global requirements"