Fix error retrieving mongodb datastore

mongodb extension is not usable on GeoServer 2.6.0.

Datastores and layers can be created, but cannot be used, throwing an error because GeoServer tries to locate the qualified name featuretype, while the existing featuretype was unqualified. Error is "Schema <namespace>:<layername> does not exist": https://github.com/geotools/geotools/blob/master/modules/library/data/src/main/java/org/geotools/data/store/ContentDataStore.java#L619

This patch propagates the workspace's namespace to mongodb DataStores and their respective FeatureTypes (Shapefile datastore has been taken as reference), so the extension works in GS 2.6.x again.

Sorry, no unit tests included. But the error is easily reproducible just trying to preview any mongodb layer on GS 2.6.