Limit the variants with config option 'tree_variants'
    
    'tree_variants' in configuration should be able to limit the tree
    variants, just like the config option 'tree_arches' which is used to
    limited the arches.
    
    For example, if there is configuration (refer to doc/configuration.rst)
    go with:
    
            tree_variants = ["Server"]
    
    then only "Server" variants will be composed, and any other variants in
    variants.xml will be ignored.
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>
    
    Rebased on master
    
    Signed-off-by: Dennis Gilmore <dennis@ausil.us>