Fix CA certificate handling

If --os-cacert was passed to the cli the following error was
triggered: "cafile must be None or a byte string". This is
because 'load_verify_locations' requires a byte string to
be passed in.

We fix this by explicitly converting the argument to a byte
string.

We do this in 'VerifiedHTTPSConnection' rather than sooner, eg
during arg handling, as it will no longer be required should we
move to a different http library (eg requests).

Fixes bug 1301849.

Change-Id: I9014f5d040cae9f0b6f03d8f13de8419597560cb

glanceclient/common/http.py

@@ -378,7 +378,7 @@ class VerifiedHTTPSConnection(HTTPSConnection):
             self.timeout = timeout
             self.insecure = insecure
             self.ssl_compression = ssl_compression
-            self.cacert = cacert
+            self.cacert = None if cacert is None else str(cacert)
             self.setcontext()
             # ssl exceptions are reported in various form in Python 3
             # so to be compatible, we report the same kind as under