osa-differ doesn't handle new roles well

[Major Hayden]

If a new role is added between two OSA releases, the script fails with an error: https://gist.github.com/evrardjp/cd5a0ae0b4453aef06bd6ed32a767d6e

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/344866

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/344866
Committed: https://git.openstack.org/cgit/openstack/openstack-ansible-ops/commit/?id=5b5ae43392cbef69a31f93a6c37e3a09331cc20f
Submitter: Jenkins
Branch: master

commit 5b5ae43392cbef69a31f93a6c37e3a09331cc20f
Author: Major Hayden <email address hidden>
Date: Wed Jul 20 09:21:47 2016 -0500

    Better handling of newton branch in osa-differ

    There was a bug in osa-differ where an exception would be thrown
    if a new role appeared that didn't exist before. That's fixed in
    this patch.

    The script also skips the commit comparison if it notices that the
    commit SHA matches between both OSA releases.

    The RST output had broken headers before and those are fixed as
    well.

    Closes-bug: 1604840

    Change-Id: Ifa47d64e7b79639a90871fdbf0370f8c425582ed