Support timezone in memcached token backend

This patch fixes bug #1221087.

Replaces time.mktime() to calendar.timegm() in utils.unixtime()
to get correct expiration time.

After this patch, Memcached token backend does not depend on timezone.
So remove the section "Memcached and System Time" in configuration manual.

DocImpact

Change-Id: I17233c8a79d01a37abdbd46934ad486ddc8caec9
Merge "Support timezone in memcached token backend"