Enable flake8 checks

This commit enables new flake8 checks:
* E265  block comment should start with '# '
* H405  multi line docstring summary not separated with an empty line
* E123  closing bracket does not match indentation of opening bracket's line
* H238  old style class declaration, use new style (inherit from `object`)
* E128  continuation line under-indented for visual indent
and makes related changes in the code.

Change-Id: Ie993afc930f6b74d7a990bcaa9fc0e9f5ba1585c

glanceclient/common/http.py

@@ -218,6 +218,7 @@ class HTTPClient(_BaseHTTPClient):
 
     def _request(self, method, url, **kwargs):
         """Send an http request with the specified characteristics.
+
         Wrapper around httplib.HTTP(S)Connection.request to handle tasks such
         as setting headers and error handling.
         """