Fix directory invalidation propagation in cluster mode

Pre-requisite:

configure 2 Nuxeo nodes in cluster (with Kafka enabled)

nuxeo.cluster.enabled=true
nuxeo.cluster.nodeid=xxx
kafka.enabled=true
kafka.topicPrefix=nuxeolts2021-

# required on the second node if same machine
nuxeo.server.http.port=9080
nuxeo.server.ajp.port=9009
nuxeo.server.tomcat_admin.port=9005

Steps to reproduce

1. on node 1, create a new user and then visit /nuxeo/api/v1/user/xxx
2. on node 2, update the first name of this new user and then visit /nuxeo/api/v1/user/xxx => observe that the first name in the JSON response is the updated one
3. on node 1, visit /nuxeo/api/v1/user/xxx => observe that the first name is the old one

Observed result : the changes are not propagated to the other node

Expected result: the changes are propagated to the other node

While investigating, we found that the provider for the invalidation in PubSubServiceImpl#publish is MemPubSubProvider whereas it should be StreamPubSubProvider

The cause is that the property "nuxeo.pubsub.provider" is still empty when kafka is enabled.

Adding nuxeo.pubsub.provider=stream to nuxeo.conf fixes the issue. Note that this is the recommended configuration at https://doc.nuxeo.com/nxdoc/kafka/#andquotno-redisandquot-nuxeo-cluster