Clean up the entire plugin

No point in having bare methods or constants outside of the class,
since nothing else uses them.

Remove unneeded log import
Remove unnecessary if/else branches
Move writing configs to a class method
Show a warning if pxe and an image path are set at the same time
Check whether a local file exists before we do anything
Move writing the config to utils.fs.File
Actually trigger screen on PXE

Change-Id: I0631c7f2f9878a3140b50a11f9a826a26798c9f7
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>