Add --signing-intent and --repo-url to 'flatpak-build'
    
    When flatpak-build was first added as a command, the signing-intent
    and repo-url parameters weren't implemented in OSBS for Flatpak builds;
    since then the Flatpak and non-Flatpak paths have been unified to a
    greater extent, making these options work. So move them to the code
    that is shared between build-container and build-flatpak.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>