Make ArtifactStore configurable

[style95]

As this PR gets merged: #4963, we have different options for ArtifactStore.
Users can use different DBs for different artifact stores.

For example, an operator can choose CouchDB as his subject and whisk entity databases and MongoDB or ElasticSearch for his activation database.
This needs to be configurable so that users can choose whatever(CouchDB, MongoDB, and ElasticSearch) he wants for each artifact store.

Additional information you deem important:

• Related discussion: Implement MongoDBArtifactStore #4963 (comment)

[jiangpengcheng]

hm, seems we need to implement additional MongoDBActivationStore and CouchDBActivationStore to achieve this feature

[rabbah]

Can’t use the elastic activation store in combination?

[jiangpengcheng]

no, it can

dominic just wants a more resilient way that users can use CouchDB serve as artifact store while MongoDB as activation store or vice versa

[rabbah]

👍 cool