Add proxy support to dynect client

Note that it is not possible to reference $http_proxy as a default to http_proxy. See https://github.com/openstack/oslo.config/blob/master/oslo/config/cfg.py#L218-L244 for more info.

Change-Id: I67ae37c74975a292d35c8a990fb4e43cd5e69d07
Closes-Bug: #1348296
Merge "Add proxy support to dynect client"