Add Unix socket DBAL parameter

| Q                   | A
| ------------------- | ---
| Bug fix?            | N
| New feature?        | Y
| BC breaks?          | N
| Tests pass?         | N
| Scenarios pass?     | N
| Checkstyle issues?  | N
| Changelog updated?  | N
| Fixed tickets       | N
| Doc PR              | N

Unix socket parameter added in config.yml and parameters.yml in order to install Akeneo on a non-default database socket.