The Zuul admin can configure authenticators with an optional
"max_validity_time" field, which is the maximum age in seconds
for a valid authentication token. By default there is no
maximum age set for tokens, except the one deduced from
the token's "exp" claim.
If "max_validity" is set, tokens without an "iat" claim will
be rejected.
This is meant as an extra security to avoid accidentally issueing
very long lived tokens through the CLI.
The "skew" field can be used to mitigate clocks discrepancies
between Zuul and a JWT emitter.
Change-Id: I9351ca016b60050b5f3b3950b840d5f719e919ce
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