Run unit tests against MySQL

- Creates a new tox environment for testing against mysql
- Adds a script set-test-env-mysql.sh to set up mysql
- Adds a new file `functions.sh` for common shell functions
- Creates a new DB manager for tests
- Adds a new scenario for mysql in the MixinTestsWithBackendScenarios

Related to blueprint sql-unit-tests-on-real-backend
Co-Authored-By: Ala Rezmerita <ala.rezmerita@cloudwatt.com>

Change-Id: I2c7378c79b1a0bffae5b2489b04c60e509f62f85