Can't use MySQL/MariaDB/Aurora as auth backend
Can't use MySQL/MariaDB/Aurora as auth backend

The current code uses SQL Alchemy to create a column of type Text and set it as a primary keys which causes the following error 

```OperationalError: (_mysql_exceptions.OperationalError) (1170, "BLOB/TEXT column 'name' used in key specification without a key length") [SQL: u'\nCREATE TABLE pypicloud_groups (\n\tname TEXT NOT NULL, \n\tPRIMARY KEY (name)\n)\n\n']```

simply changing all references of `Text()` to `String(length=255)` seems to fix this.
mathcamp/pypicloud,mathcamp/pypicloud,mathcamp/pypicloud,rubikloud/pypicloud,stevearc/pypicloud,mathcamp/pypicloud,stevearc/pypicloud,stevearc/pypicloud,rubikloud/pypicloud,rubikloud/pypicloud,rubikloud/pypicloud,stevearc/pypicloud