Revert "change the default to PyMYSQL"

The failure rate with neutron is too high to keep this
as the default.

Related-Bug: #1464612

This reverts commit b3798af.

Change-Id: Ie9550aeb25d472a38e3d3ef6f3711622c9221c46

lib/databases/mysql

@@ -11,7 +11,7 @@
 MY_XTRACE=$(set +o | grep xtrace)
 set +o xtrace
 
-MYSQL_DRIVER=${MYSQL_DRIVER:-PyMySQL}
+MYSQL_DRIVER=${MYSQL_DRIVER:-MySQL-python}
 # Force over to pymysql driver by default if we are using it.
 if is_service_enabled mysql; then
     if [[ "$MYSQL_DRIVER" == "PyMySQL" ]]; then