Allow primary metastore to have a prefix

[AnanaMJ]

As a user
I'd like to be able to set primary_metastore.database-prefix in the configuration without it being replaced by empty string
So that I can apply consistent prefixes across multiple waggle dance instances having different primary and federated metastores and thus re-use the same queries regardless of which waggle dance instance I am connected to.

[massdosage]

Depending on what we do for #154 this might be resolved at the same time but it could also be worth doing sooner as this is already a feature that would be useful in Apiary.