Proxy Support not present in tempest framework

[Ankit Arora]

Tempest framework currently uses urllib3 PoolManager to invoke api calls. This works in all cases except in those where the api calls have to be routed via a proxy server. There might be many usecases for routing api calls via a proxy. One of them is routing the calls to a proxy server(on a particular port) where a security scan tool is listening to these api calls. Support for routing calls to a proxy server is available in python requests library where it can either be set via a environment variable (HTTPS_PROXY) or send as an input api parameter. However, tempest framework does not use requests library yet.

urllib3 library has a class named ProxyManager (exactly same as and inherited from PoolManager but with proxy support). This can be used instead of PoolManager. Tempest users can be given an option to input proxy ip and port via a config attribute.

https://github.com/openstack/tempest/blob/master/tempest/lib/common/http.py#L20