Merge pull request #147 from jkoncick/master

Fix rpm installation in RHEL, print pretty messages when there are no assistants.
Remove intermediate docker images
Add a note about result of Python bool values
Document how to use an argument with no parameters
Fix handling of arguments with 'nargs: 0' by GUI
Add store_const to recognized 'bool' parameters for GUI
Fix setting default values for assistant/snippet attributes.
Related: https://bugzilla.redhat.com/show_bug.cgi?id=1059305
gui: Do not ask for project name when deps-only is selected
gui: Fix removing kwarg when user untick check button
gui: Fix checking whether textual parameter value is empty