Since the chef-client provisioner is cleaning the node and client at the chef-server from the provisioned node
it needs to have a flexible configuration

This is replacing the used knife flags: -s '<chef-server-url>' -k '/tmp/packer-chef-client/client.pem' -u '<client-name>'
and puts their values into a generated knife.rb

Additionally the knife.rb may include the optional ssl_verify_mode attribute to enable the verify mode verify_none

Background:

When deleting node and client to a self-hosted chef-server using self signed cerfiticates the usage of

    knife node delete <node-name> -y -s '<chef-server-url>' -k '/tmp/packer-chef-client/client.pem' -u '<client-name>'

will lead into a ssl verification failure.

The error output of the knife call is somthing like:

    2015/06/24 12:29:17 ui:     docker: WARNING: No knife configuration file found
    docker: WARNING: No knife configuration file found
    2015/06/24 12:29:17 ui:     docker: ERROR: SSL Validation failure connecting to host: 172.16.117.63 - SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B:
    certificate verify failed
    docker: ERROR: SSL Validation failure connecting to host: 172.16.117.63 - SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed
    2015/06/24 12:29:17 ui:     docker: ERROR: Could not establish a secure connection to the server.
    docker: ERROR: Could not establish a secure connection to the server.
    2015/06/24 12:29:17 ui:     docker: Use 'knife ssl check' to troubleshoot your SSL configuration.
    docker: Use 'knife ssl check' to troubleshoot your SSL configuration.
    2015/06/24 12:29:17 ui:     docker: If your Chef Server uses a self-signed certificate, you can use
    docker: If your Chef Server uses a self-signed certificate, you can use
    2015/06/24 12:29:17 ui:     docker: 'knife ssl fetch' to make knife trust the server's certificates.
    docker: 'knife ssl fetch' to make knife trust the server's certificates.
    2015/06/24 12:29:17 ui:     docker:
    docker:
    2015/06/24 12:29:17 ui:     docker: Original Exception: OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed
    docker: Original Exception: OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed
    2015/06/24 12:29:17 packer-builder-docker: 2015/06/24 12:29:17 Executed command exit status: 100