Inconsistent timestamp format for different datasources

[Ifat Afek]

Different datasources report their updated_timestamp in different format. The format of sample_timestamp may also be different.

An example can be seen here: https://docs.openstack.org/vitrage/latest/contributor/notifier-webhook-plugin.html

See also: http://lists.openstack.org/pipermail/openstack-dev/2018-June/131327.html

Expected behavior: the format should be '%Y-%m-%dT%H:%M:%SZ' as defined in https://github.com/openstack/vitrage/blob/master/vitrage/datasources/transformer_base.py