Use flagx.StringFile for autocert.hostname and token.machine

[stephen-soltesz]

This change updates the flag type used by the -autocert.hostname and -token.machine flags to flagx.StringFile. This flag type allows a single flag to pass a literal string value, e.g. the actual hostname, or a filename with the contents for the string value, e.g. the hostname in a file.

This change is in service of enabling access token verification in autonode deployments. Current users of autocert.hostname will need to be updated with a prefix of @ to read from the named file. The only known users are from https://github.com/m-lab/autonode

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This change is 

[robertodauria]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained