Use strtime() specific timestamp regexp

Any datetime objects that get serialized via jsonutils.dumps() get
stringified using strtime() which includes decimal seconds and is
timezone naive.

Use a specific regexp to check the use of this format.

Note that included in here is a fixup to the doc samples for the
instance actions extension - the sample shows it using the
str(datetime) format when in fact it is using strtime. This came
about because before commit 68288b9 we were using a pre-serialized
timestamp in the fake instance actions. I just regenerated the samples
for this.

Full context here:

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

Change-Id: If52a2a664eccc8aed8a39d1a9dfb0209337c3c79
Merge "Use strtime() specific timestamp regexp"