sslutil: change comment and logged message for found ca cert file
Future patches will change _defaultcacerts() to do something
on platforms that aren't OS X. Change the comment and logged
message to reflect the future.
tests: better testing of loaded certificates
Tests were failing on systems like RHEL 7 where loading the system
certificates results in CA certs being reported to Python. We add
a feature that detects when we're able to load *and detect* the
loading of system certificates. We update the tests to cover the
3 scenarios:
1) system CAs are loadable and detected
2) system CAs are loadable but not detected
3) system CAs aren't loadable