Normalize API extension updated timestamp format

It's unusual to include a '+00:00' offset in an ISO8601 timestamp
rather than just using the 'Z' suffix. It's also very weird for our
API to be returning timestamps which aren't in UTC.

Let's make these timestamps consistent with other timestamps by
using UTC always and representing that with a 'Z' suffix. Also,
enforce this in the API sample tests by using a new 'isotime' regexp.

A small number of the extensions in the API sample templates
specified the exact timestamp, so templatize those before regenerating
the API samples for GET /extensions.

Full context here:

  http://lists.openstack.org/pipermail/openstack-dev/2014-April/033971.html

Change-Id: Idf429e55e4ae13738ac531a25ce54b20d395410d
Merge "Normalize API extension updated timestamp format"