Definition: fix loadToCache to return `id`

Fix the bug introduced by 47837ff where `Definition.loadToCache`
no longer returned the id of the created instance. That caused
`Facet.loadIntoCache` to throw because facetId was undefined.
DataSourceDefinition: prevent double callback
Merge pull request #120 from strongloop/feature/fix-tests

Another attempt to fix unit-tests failing on CI