Add ca_certificates_file option to test against SSL servers

tempest has an option to ignore SSL errors
(disable_ssl_certificate_validation) but no way to pass in
a CA bundle to be used to do actual validation. This new option,
ca_certificates_file, will provide this bundle.

boto testing will be handled separately as it gets CA
certificates from its configuration file.

This is configured in either /etc/boto.cfg or the file pointed
to by $BOTO_CONFIG via:

[Boto]
ca_certificates_file = /path/to/ca-bundle.pem

Change-Id: I5662e881f7d71ed34a8fe4b9d0a2541a6ff1bad3
Closes-bug: #1395754