transmit optional mongoDB params

Transmitting the optional params to ensure that the protected databases can be accessed.
Consult the PHP documentation for more information: http://php.net/manual/de/mongoclient.construct.php

cache.xml example:
```xml
    <backend>
        <type>Pimcore_Cache_Backend_Mongodb</type>
        <custom>true</custom>
        <options>
            <dbname>dbName</dbname>
            <optional>
                <db>dbName</db>
                <username>dbUser</username>
                <password>dbPassord</password>
            </optional>
        </options>
    </backend>
```