Install PyMySQL if used

Change Ic609ce136061b753ca692b37509a0b29c60bb8b5 switched to PyMySQL by
default but does not make sure it is installed. This is causing gate
failure in Gnocchi for example:

  http://logs.openstack.org/25/186025/3/check/gate-gnocchi-dsvm-functional-file-mysql/eebd773/logs/devstacklog.txt.gz

Change-Id: I23d313220607fcc8acb95ab43f55b7d9899b9b1f

lib/databases/mysql

@@ -165,6 +165,8 @@ function install_database_python_mysql {
     pip_install_gr $MYSQL_DRIVER
     if [[ "$MYSQL_DRIVER" == "MySQL-python" ]]; then
         ADDITIONAL_VENV_PACKAGES+=",MySQL-python"
+    elif [[ "$MYSQL_DRIVER" == "PyMySQL" ]]; then
+        ADDITIONAL_VENV_PACKAGES+=",PyMySQL"
     fi
 }