Make cli "roles create" idempotent

[fenekku]

invenio roles create <your role name> fails if re-run, but the goal of the cli command is just to have a '' role entry in the DB. Checking for existence and only creating if not there, would not break compatibility and make usage of the command much smoother. This just means using flask_security.datastore.UserDataStore.find_or_create_role instead. (I will probably send a PR)

This request stems from invenio roles create admin being used in instance setup where the goal is to get to a state rather than execute a particular action.