Add hint to the tab 'Storage Backends'

Add hint to the tab 'Storage Backends' with the following text:
'VMDK driver is a default option for vCenter. Please change a Cinder
LVM option to a Cinder VMDK option in the environment settings.'

Change-Id: Ifd7737c3bc8b9675e68888adb696ce75abe90e9e
Closes-bug: #1359696
Merge "Add hint to the tab 'Storage Backends'"