Remove iso8601 dependency

[OpenStack Infra]

https://review.openstack.org/249596

Dear documentation bug triager. This bug was created here because we did not know how to map the project name "openstack/mistral" to a launchpad project name. This indicates that the notify_impact config needs tweaks. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 909ce2934ef47758e82731c1cbcf57a13a25f3b8
Author: Bertrand Lallau <email address hidden>
Date: Mon Nov 16 16:24:27 2015 +0100

    Remove iso8601 dependency

    Mistral does not import and use this module directly, no need to list
    it in the requirements and no need to set debug level for this module.

    DocImpact

    Change-Id: I42d5661dce0d23777fc5801c20a3a7c86756007d
    (cherry picked from commit 91f35ae9fb8d4efc042baa3028f7c7369219d280)