Handle host side SSL certificates validation

 - add --os-cacert option which allows to set a file containing
   certificates of root CAs (certificate authorities) that are
   required for validation of HTTPS servers SSL certificates

 - wrap httplib2 SSL certificates validation errors with a
   custom quantumclient exception

Blueprint: quantum-client-ssl

Change-Id: I4e6a7d177ba14314ba9bed613ec2684bffc35222
Merge "Handle host side SSL certificates validation"