Convert IOError from requests

This requests commit [1] changed the behavior when a nonexistent cacert
file is passed in: now it raises IOError.  This is getting through
glanceclient.common.http.HTTPClient._request, which used to raise
CommunicationError in this scenario.

Even though there is arguably a better exception than CommunicationError
to represent this condition (like maybe IOError), for backward
compatibility this change set converts IOError to CommunicationError.

We also improve the unit test to raise the original exception if the
expected conditions aren't met; this improves debugability.

[1] https://github.com/kennethreitz/requests/commit/7d8b87c37f3a5fb993fd83eda6888ac217cd108e

Change-Id: I6a2cf4c6d041b67d3509153b4cef18b459263648
Closes-Bug: #1692085

glanceclient/common/http.py

@@ -257,7 +257,7 @@ class HTTPClient(_BaseHTTPClient):
             message = "Error finding address for %s: %s" % (
                 self.endpoint_hostname, e)
             raise exc.InvalidEndpoint(message=message)
-        except (socket.error, socket.timeout) as e:
+        except (socket.error, socket.timeout, IOError) as e:
             endpoint = self.endpoint
             message = ("Error communicating with %(endpoint)s %(e)s" %
                        {'endpoint': endpoint, 'e': e})