Use PyMySQL in MySQL related functional/fullstack tests

Use PyMySQL in MySQL related functional/fullstack tests

mysql-python driver has been replaced by PyMySQL driver[1] in neutron
code but MySQL related functional/fullstack tests try to use
mysql-python driver because of MySQLOpportunisticTestCase[2] and tests
are skipped because mysql-python driver is no more available.

This change provides a backend implementation for mysql+pymysql, a base
base testcase MySQLTestCase[2] using mysql+pymysql implementation
(currently oslo.db provides none of them but will in the future) and
replaces MySQLOpportunisticTestCase with MySQLTestCase.

[1] I73e0fdb6eca70e7d029a40a2f6f17a7c0797a21d
[2] neutron.tests.common.base

Closes-Bug: #1463980
Change-Id: Ic5c1d12ab75443e1cc290a7447eeb4b452b4a9dd