Storwize/SVC: fix attach bug for live migration.

Add force flag for mkvdiskhostmap command to allow multi host mapping

Change-Id: I290a5986b213a0aaeae0d0ea0ad57350c9dee3c9
Fixes: bug #1179773
(cherry picked from commit 88c8b42194333566c085489b160d77fc71b61b86)
Merge "Storwize/SVC: fix attach bug for live migration." into stable/grizzly