Replace usage of 'retrying' with 'tenacity'

[Boden R]

Today a number of OpenStack projects use the 'retrying' library [1] for generic retry behavior. While retrying provides a functional API, its author no longer actively maintains the repo and hasn't responded to numerous PRs, emails, etc. (more discussion in [2]). As a result, we can't push fixes/features to retrying to support various initiatives such as [3].

A fellow stacker graciously forked the retrying repo and revamped it's API to provide greater functionality/pluggablility; called tenacity [4]. While tenacity provides the same functionality as retrying, it has some notable differences such as:
- Tenacity uses seconds rather than ms as retrying did.
- Tenacity has different kwargs for the decorator and
Retrying class itself.
- Tenacity has a different approach for retrying args by
using classes for its stop/wait/retry kwargs.
- By default tenacity raises a RetryError if a retried callable
times out; retrying raises the last exception from the callable.
Tenacity provides backwards compatibility here by offering
the 'reraise' kwarg.
- Tenacity defines 'time.sleep' as a default value for a kwarg.
That said consumers who need to mock patch time.sleep
need to account for this via mocking of time.sleep before
tenacity is imported.
- For retries that check a result, tenacity will raise if the retried
function raises, whereas retrying retried on all exceptions.

We'd like to move from retrying to tenacity and eventually remove retrying from global requirements all together.

For projects using retrying, the move to tenacity (hopefully) isn't overly intrusive, but must take the above differences into consideration.

While I'm working to move all affected projects [6] from retrying to tenacity, this effort is a work in progress (under [5]).

[1] https://github.com/rholder/retrying
[2] https://review.openstack.org/#/c/321867/
[3] http://lists.openstack.org/pipermail/openstack-dev/2016-April/092914.html
[4] https://github.com/jd/tenacity
[5] https://review.openstack.org/#/q/message:%22Replace+retrying+with+tenacity%22
[6] http://codesearch.openstack.org/?q=retrying&i=nope&files=.*.txt&repos=

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/390405

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/390405
Committed: https://git.openstack.org/cgit/openstack/taskflow/commit/?id=1a2370ac917ed1f888ef98a45270912b08f27844
Submitter: Jenkins
Branch: master

commit 1a2370ac917ed1f888ef98a45270912b08f27844
Author: Gevorg Davoian <email address hidden>
Date: Tue Oct 25 00:22:12 2016 +0300

    Replace retrying with tenacity

    This patch replaces the legacy retrying library with the newer
    and more convenient tenacity one, taking into account that:
    1) retrying uses milliseconds for wait times, but tenacity uses seconds;
    2) retrying has a lot of numeric arguments for specifying behaviour
    of decorated functions, while tenacity has a few of them, which are
    specialized objects, thus making the retry-decorator more flexible.

    Change-Id: I4b165d37b2ecc210f2b94c103b73eaab51529261
    Closes-Bug: #1635404

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/taskflow 2.8.0 release.