add ability to specify individual parameters on cmdline without JSON

This should allow most command line uses of the parameters to occur
without the user having to generate a JSON file to be passed in the
--parameters argument.

For example, a kickstart can now be specified to the base_image generation
like this:

imagefactory base_image --parameter-file install_script kickstart.ks input.tdl

Or icicle generation can be disabled like this:

imagefactory base_image --parameter generate_icicle 0 input.tdl

TODO: Have the plugins specify what parameters they accept and what the default
      values are.