Revert "Revert "change the default to PyMYSQL""

Some projects (Neutron) seem to be affected more than others, so we should revert this
to allow for a more selective choice of the DB driver on a per project basis.

We can re-enable the use MySQL-python just for Neutron.

This reverts commit de8d29e.

Related-Bug: #1464612

Change-Id: I889f4f8b116c413b300ab9eecc7b428a9a4afb1a

Author: armando-migliaccio

lib/databases/mysql

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