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: refs/heads/master@{#336739}