Support non-UTC timestamps in changes-since filter
Support non-UTC timestamps in changes-since filter

Fixes nova aspect of lp 837464

Prevously only Zulu time was supported in the changes-since filter,
i.e. timestamps formatted as %Y-%m-%dT%H:%M:%SZ

We now support arbitrary timezones, with the offset from UTC expressed
via the ISO 8601 ±hh:mm notation.

Microsecond accurracy is also optionally supported in timestamps.

Notes:

- nova.utils.parse_isotime(), isotime() & normalized_time() are
  prime candidates for promotion to openstack-common, as these
  methods were duplicated from my corresponding glance patch:

    https://review.openstack.org/#change,4198

- this patch introduces a new dependency on python-iso8601, which
  has already been packaged for Fedora, EPEL and Ubuntu/Debian.

Change-Id: I89b45f4f3d910606c578d927420f78cea94f4e3b

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects