Use system supplied certs without a bundle file

- Allow the use of system provided certificate setup that may be incorporated

  into the SSL library used on the specific system
  + At present we either use the default certificate bundle we ship with the
    boto source, or we force a user/integrator to create a bundle file of their
    own. Linux distributors build the way certificates are used and validated
    into their SSL implementation. This change allows integrators to use their
    way of certificate handling by setting the configuration to the new
    "system" keyword.