Fix for "No existing metadata" in Horizon metadata selector.

In path 'Admin' -> 'Image' -> 'Update Metadata'

If all metadata items are added to "Existing metadata",
"No existing metadata" will be shown in "Available Metadata" menu.

This patch replace "No existing metadata" string to "No available
metadata", to make situation clear.

Change-Id: Ie04ca6c444e149860ccb6fc059fad3d770c6631f
Closes-Bug: 1369248
Merge "Fix for "No existing metadata" in Horizon metadata selector."