Disambiguate unit test and preview K8s namespaces

Currently, these namespaces have the following patterns:

nuxeo-unit-tests-2021-114-postgresql
nuxeo-unit-tests-master-870-postgresql

nuxeo-preview-master
nuxeo-preview-2021

When running a build from the staging environment, there can be some collisions on the unit test namespaces, and the preview namespace is always overridden.
Let's disambiguate these namespaces by prefixing them with the current namespace: platform or platform-staging.

Moreover, this will allow to better categorize these namespaces in the cluster.