Change default for cert_required

This setting is used to require client certificates, which people likely
do not want by default.
Add an attribute to let the user say the certificate is insecure
webui: Fix setting cert_required
Link ca_certs attribute to cert_required in the UI

If cert_required is false, then ca_certs will not really matter.
Fail early if certificate files are missing
Merge pull request #94 from vuntz/ssl-fixes

https://github.com/crowbar/barclamp-keystone/pull/94

Merged by devtool for cloudsleuth