fall back to SHOW VARIABLES for MySQL < 5.6

fall back to SHOW VARIABLES for MySQL < 5.6

Fixed regression caused by :ticket:`7518` where changing the syntax "SHOW
VARIABLES" to "SELECT @@" broke compatibility with MySQL versions older
than 5.6, including early 5.0 releases. While these are very old MySQL
versions, a change in compatibility was not planned, so version-specific
logic has been restored to fall back to "SHOW VARIABLES" for MySQL server
versions < 5.6.

includes unrelated orm/test_expire ordering issue , only showing
up on 1.4 / py2.7 but seems to be passing by luck otherwise

Fixes: #7518
Change-Id: Ia554080af742f2c3437f88cf3f7a4827b5e55da8