Implement a https:// in REST alarm notification

This change adds schemes https:// to the alarm notifier.

By default, the server certificate is verified like a browser does
but with the system CA.

A client certificate can be set in the configuration file.

 [alarm]
 rest_notifier_certificate_file = /path/certificate.pem
 rest_notifier_certificate_key = /path/private_key.pem

Change-Id: Id06e0a45ef88c15674052faeb941d87b70c7b99b
Blueprint: alarm-notifier