Convert revocation list file last modified to UTC

On a restart of a service using auth_token middleware, the last modified
time of the revocation list file is checked to decide whether to get the
fresh list from keystone. In server timezones that are ahead of UTC,
this compares a local time with UTC. This means whenever a service is
restarted it doesn't update the revocation list for the length of the
timezone offset from UTC.

This change converts the last modified time to UTC when it's first
read, so the comparison is valid.

Closes-bug: 1204000
Change-Id: I623b6273beb56f8da2a8649a10a64318da8cd6bc
Merge "Convert revocation list file last modified to UTC"