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
fixed errors when run_tests.sh --makemessages

The change Ibf2036d5bbc4c478fcb912f6335104356e8f1317 causes
run_tests.sh --makemessages failure. Make the errors go way.

Closes-Bug: #1379054
Change-Id: I66b68099561bdd451321c7c31391f30827121e7c
Merge "Fix for "No existing metadata" in Horizon metadata selector."
Merge "fixed errors when run_tests.sh --makemessages"