config: Validate variant regular expressions
    
    When the mapping in configuration specifies incorrect regular expression
    to match regular expressions, we should raise an error immediately and
    not wait until the part of config is actually used.
    
    This patch does not cover `live_media`, `image_build` and `osbs`
    sections, as they use plain dicts and not the list of tuples format.
    
    Fixes: #424
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>