Fixes cinder volume delete on Windows

Because of the fact that an iSCSI target is not created anymore
at volume creation time, not all volumes will have a corresponding
target. For this reason, when removing an iSCSI target we must
first check if the target actually exists.

Change-Id: I8e571397df6d9a2eeb05e883b7da8494d4bfa1ad
Closes-Bug: #1299124
(cherry picked from commit 601d54c9ef926fa2d8abfe0121019f7ad94eefd5)
Merge "Fixes cinder volume delete on Windows" into milestone-proposed