policy_testserver.py: Optionally resolve username.
policy_testserver.py: Optionally resolve username.

This enables policy_testserver.py to resolve the user name from the
auth token provided by the client when making a registration call. The
server does this when a token_info_url parameter is specified in the
configuration file. Hard-configured user names continue to function in
the absence of the token info endpoint.

BUG=None
TEST=compiles and passes tests

Review URL: https://codereview.chromium.org/1210063003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336739}

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