Fallback to primary metastore when no prefix

[massdosage]

In #152 we added the feature where the primary metastore can have a prefix. However this change will not be backwards compatible if we enable this for an existing Apiary installation where users are used to having their queries succeed against the primary without having a prefix. There might be workarounds we can do with configuring the metastore multiple times with and without a prefix but this will lead to duplicate tables etc. being returned.

Ideally we want this situation to prevail:

• standard name is with prefix in all cases - i.e. all calls which return values have the prefixed version and the non-prefixed version does not need to be returned.
• backward compatible feature is this: “if query does not have prefix then WD defaults to using the prefix associated with the primary metastore”. That does require all current WD implementations to do a 1 time update of their configs when deploying this feature (as prefix in config for primary metastore is required). However, one change, one notice to users, automatic backward compat, and things are standardized from then on.

Given that at some point in the future we want to remove the concept of the primary metastore (see #154) perhaps it would be worth introducing a flag that you can put on the primary metastore like fallback-metastore=true or fallback-prefix="" so that it can be put on any metastore but only one per WD instance.