engine: Rename QueryPrefix to QUERY_SUFFIX

The attribute name should be change to reflect its
nature (a suffix of a query class name) and to match
java naming standard.

Change-Id: I8db1fff2b8c7bb23e0bc0c6fbe5576b7199a7b8d
Signed-off-by: Moti Asayag <masayag@redhat.com>