In v1.20, TokenRequest and TokenRequestProjection become GA features,
and the following flags are required by the API server:
* --service-account-issuer
* --service-account-key-file
* --service-account-signing-key-file
This change ensures that the flags are set, and that the required keys
are in the right places.
Change-Id: I6606c5b1c9ff005d1943b424e3e7ad4d20b68408