[STORAGE] allow use of python-mysql.connector

Debian Wheezy doesn't have pymysql package, but seems that all the
functionalities needed are into `python-mysql.connector`.

NOTE: I don't have updated these files (and probably we don't need to):
- `pip-requires.txt`
- `py2exe_builder/readme.txt`
- `tests/storage/test_DatabaseStorage.py`