Fix wsgi/SSL/ipv6 tests for wsgi module under python 3
Fix wsgi/SSL/ipv6 tests for wsgi module under python 3

Previously some tests were ignored under python 3 environment, this was
due to some design changes introduced by python 3.7 [1] in the SSL
module of the stdlib. These changes reactivate some of them (some other
are still skipped and needs further works).

Indeed, when we try to use requests with SSL in a monkey patched
environment we faced the following issue:

```
TypeError: wrap_socket() got an unexpected keyword argument '_context'
```

This is due to the fact that we are in a monkey patched environment
where `requests` is monkey patched too.

We don't need `request` for our needs. Indeed we can easily send
http requests through low level socket. Our main goal is to test
our wsgi server and not to test the `requests` library, and `requests`
was just used to make the code more simpler.

In our case we can implement a code dedicated to send request to our green
server, unlock our tests and move away from this bug/side effect.

These changes move away from `requests` which is badly monkey patched by
eventlet [1]. Now we use monkey patched socket and ssl to
request the green server which is executed in background. Low level
(monkey patched) modules could help us to skirt layers that are possibly
badly monkey patched on higher level modules (urllib, requests, etc...).

[1] https://github.com/eventlet/eventlet/issues/526#issuecomment-482694279
[2] https://github.com/eventlet/eventlet/issues/526

Change-Id: Iacdde51d2d923bafa3263fa3dc12de8d501d471a
Partial-Bug: #1482633

openstack/oslo.service