Add specific regexp for timestamps in v2 xml

datetime objects are serialized into xml using simply str() and this
is a slightly different format from ISO8601 in that the date isn't
separated from the time using 'T'.

(However, note that the iso8601 library happily accepts this format)

Add a specific regexp for this format so we can test for it in the
places we know it is used. This also means we can remove the generic
%(timestamp)s regexp.

Note that unlike the isotime and strtime formats, whether this format
includes timezone or microsecond information depends on whether the
datetime object had those fields set. The isotime format always
includes a timezone but not microseconds, whereas the strtime format
never includes a timezone but always includes microseconds.

There are a small number of examples where this format is used in JSON
too - e.g. the instance usage audit log extension pre-serializes its
timestamps by doing:

        return dict(period_beginning=str(begin),
                    period_ending=str(end),

Using a name like 'xmltime' for the timestamp format used in cases
like this actually makes sense - it highlights that the format used
in this case is a weird mistake.

Full context here:

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

Change-Id: I70f839ac17273ed10078b833aeba308bd5e994e1