Remove timestamp types from resource properties

[Richard Theis]

Timestamp types were added to resource properties under https://bugs.launchpad.net/python-openstacksdk/+bug/1544584. However, these types can cause problems as reported by https://review.openstack.org/#/c/316700/. This bug will remove the timestamp types from the resource properties. We may be able to simply revert the patches merged under https://bugs.launchpad.net/python-openstacksdk/+bug/1544584.

Table to track overall progress by component
    [TODO] block_store
    [TODO] cluster
    [TODO] compute
    [No changes] database
    [TODO] identity
    [TODO] image
    [TODO] key_manager
    [No changes] message
    [TODO] metric
    [TODO] network
    [TODO] object_store
    [TODO] orchestration
    [TODO] telemetry

[Brian Curtin (brian.curtin)]

This was fixed in https://review.openstack.org/#/c/327315/