Allow to configure the Chef Server certificate

Starting from version 12, the Chef client has the SSL verification enabled by default. That makes the current deployments to fail, as the client is not configured with the path to the Chef server certificate to use for validation.

An option should be added to the BootstrapConfig object so users can provide the server certificate at config time. This option should be the contents of the certificate file, and the bootstrap process will write it in an appropriate location and configure the Chef client to use that file.