webadmin: Correction of UI Constant

Detach Storage dialog text related issues solved: "Data Center(s)"
was changed to "Data Center", "Detach storage" was changed to "Detach
Storage"

Change-Id: I8a0848956c472fae929e36841de65d893e728d96
Bug-Url: https://bugzilla.redhat.com/1149457
Signed-off-by: Amit Aviram <aaviram@redhat.com>