Default to system CA bundle if no CA certificate is provided

If no CA certificate is provided to be used for validation then
fall back to the system-wide CA bundle as presented by
python-requests.

Change-Id: I05206a868150d4b62b6f1b833310eb9b86b7c4f8
Closes-bug: #1362179
Merge "Default to system CA bundle if no CA certificate is provided"