* 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;
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