mongorestore assumes admin.system.version only contains authSchema document

In GetDumpAuthVersion(), we try to read the authSchema document from the admin.system.version collection by reading the first document from the collection. In SERVER-25155, we are adding a new document called featureCompatibilityVersion to the admin.system.version collection. Then if the first document in the collection is featureCompatibilityVersion, we will think it is the authSchema document and fail here. Instead of reading the first document from the collection, we should check that _id is 'authSchema'.

For example, SERVER-25155 will cause this test to fail. To commit SERVER-25155, we will need to ban this test until this issue is fixed.