Quota column name 'key' in downgrade script

006_mysql_downgrade didn't correctly quote 'key' column of
'image_properties' table when creating index, the index wasn't created
as needed, and then operator could not run any upgrade script
successfully since 006_mysql_upgrade script could not to handle this
unexpected db status.

Note, this issue doesn't impact one-way upgrade migration operation,
since bug existed in downgrade script. But once operator do a
reciprocating migration operation, this issue will rise up as a blocker.

Closes-bug: 1351413

Change-Id: Ie1ed1b7f06b8a0aeede70c3d4dd061c857e448da
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Quota column name 'key' in downgrade script"