Add env qa command group

[ofek]

The next PR will add usage documentation to the Agent repo's site.

A good way to test would be to

1. Run dda env qa start to start the linux-container QA environment
2. Run dda env qa run agent status to confirm it's working
3. Run dda env qa config find to locate the datadog.yaml file and modify its contents in some way
4. In the directory that contains the datadog.yaml file (also can open directly in your file manager via the dda env qa config explore command) add an integrations directory with a subdirectory postgres containing a conf.yaml file (or any file ending in .yaml/.yml) with valid contents like:

   instances:
   - name: foo

5. Run dda env qa config show to confirm your changes appear correct
6. Run dda env qa config sync to update the configuration used by the Agent
7. Run dda env qa run agent status to confirm that the postgres integration is now running
8. Run dda env qa stop -r to stop and remove the QA environment (or the more involved dda env qa stop followed by dda env qa remove)