Bring SalsaFlow config format up to date

Replace .salsaflow.yml with the metadata directory .salsaflow.

Change-Id: 4131ce1be8
Implement the model for Pivotal Tracker

The model for Pivotal Tracker should be up to date now, in sync with the
latest SalsaFlow changes.

Also the module uses a different PT client library that uses API v5
instead of v3.

Change-Id: 111f270cd5
pivotaltracker: Fix review issues

Change-Id: d0db86c8e3
pivotaltracker: tests: Use CS multiline strings

Change-Id: a4af928adb
pivotaltracker: tests: Fix description templates

Change-Id: a92bf0142d
Merge pull request #1 from salsita-integrations/story/pt-review-post-hooks

Implement the issue tracker interface for Pivotal Tracker