certsdb sqlite data_source should be relative to db-config

With the following directory structure:
```
.
├── ca
│   ├── cert.csr
│   ├── cert-key.pem
│   ├── cert.pem
│   └── request.json
├── certs.db
├── db-config.json
└── example.com
├── cert.csr
├── cert-key.pem
└── request.json
```

If I'm in the `example.com` directory, and want to generate a certificate with:
```
cfssl sign -ca ../ca/cert.pem -ca-key ../ca/cert-key.pem -config ../../config.json -profile server -db-config ../db-config.json cert.csr
```

The db-config.json file must have the following:

```
{"driver":"sqlite3","data_source":"../certs.db"}
```

Ideally certs.db would be relative to db-config.json though, then the db-config.json file could be:

```
{"driver":"sqlite3","data_source":"certs.db"}
```