Reuse of valid token on authorization by default

I'd like to propose the following for token reuse for PKI and PKIZ token types. As there is always a concern of tokens growing and keystone-manage failing to cleanup if the transaction timeout happens. Database will keep on growing and eventually create problems.

Capability to:
1. Reuse of unexpired (valid) token on authorization request by default for persistent tokens.
2. Additional attribute can be sent to request for a generating a new token.