oslo_utils.timeutils.isotime() is deprecated as of 1.6 so we need
to stop using it.
The deprecation message says to use datetime.datetime.isoformat()
instead, but the format of the string generated by isoformat isn't
the same as the format of the string generated by isotime. The string
is used in tokens and other public APIs and we can't change it
without potentially breaking clients.
So the workaround is to copy the current implementation from
oslo_utils.timeutils.isotime() to keystone.common.utils.isotime().
Change-Id: I34b12b96de3ea21beaf935ed8a9f6bae2fe0d0bc
Closes-Bug: 1461251
97c2c690d8