kubernetes: create a namespace associated to each user

[diegodelemos]

Once a user is logged in REANA for the first time they will be associated with a Kubernetes namespace. All resources belonging to the concrete users will be stored in their separated namespace.

[tiborsimko]

For REANA 0.7.0, we have introduced support for running REANA in a dedicated namespace. The topic of using multiple namespaces for multiple users will be addressed as part of the OIDC/RBAC sprint coming later.