Vault role updates:

  * using separated vault roles for generate certs with different `O` (Organization) subject field;
  * configure vault roles for issuing certificates with different `CN` (Common name) subject field;
  * set `CN` and `O` to `kubernetes` and `etcd` certificates;
  * vault/defaults vars definition was simplified;
  * vault dirs variables defined in kubernetes-defaults foles for using
  shared tasks in etcd and kubernetes/secrets roles;
  * upgrade vault to 0.8.1;
  * generate random vault user password for each role by default;
  * fix `serial` file name for vault certs;
  * move vault auth request to issue_cert tasks;
  * enable `RBAC` in vault CI;