v2.1.4: [patch] Use env sa prefix (#76)

fix mapping for service account without namespace

old: system:serviceaccount:user => athenz.domain._env_._namespace_.user
new: system:serviceaccount:user => athenz.domain.dev.empty-namespaces.user

# SubjectAccessReview request body
{
  "user": "system:serviceaccount:user",
  "group": [
    "system:serviceaccounts"
  ]
}

# garm-config.yaml
empty_namespace: empty-namespaces
athenz_service_account_prefix: athenz.domain._env_._namespace_.

# garm-extapi.yaml
env: dev