Ability to define a suffix for the Raven client service

Instead of using a big ugly `sha1($dsn)` suffix to define the Raven client service, we use a readable/easily writable suffix.

Before:

    monolog.raven.client.4b9d0f59c45d393c0fc2ac65d6f07ff1b71954cd

After:

    monolog.raven.client.prod