Add ignore-ssl and ca-certs options to CLI

Refs #54

Both options are set either in the config file or as runtime
arguments.

ignore-ssl option allows connections without checking SSL
certificates

The ca-certs option provides a CA file for SSL authentication

Normally, if no cert files are provided, kamaki attempts to connect
anyway, and gets an SSL Error.

In case of an SSL error, kamaki stops and notifies the user
except if they use the ignore-ssl option. Even with ignore-ssl,
kamaki will always attempt to use the CA file, if available.

The default value of the CA file is set in the file kamaki/default_certs.py
The only purpose of this file is to simplify setting the CA certifications
path. It should be modified or generated by the package manager, this it
contains only one line of code.
Update documentation about SSL setup

Fixes #54