token: revoke expired tokens

[greut]

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Relates #740

When a token is expired, it may still exist in the Vault but with an expire_time set to null. This change aims a not failing, neither panicing (#740), but revoke (when possible) and remove that token from the state.

Release note for CHANGELOG:

Expired Vault tokens might still exist and will have `expire_time` set to nil. In that case, they will be revoked (when possible) and removed (always) from the local state. Expired token will then be recreated by Terraform.

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[hashicorp-cla]

All committers have signed the CLA.