Block retiring in released branches
    
    It checks Fedora release state and it restricts retiring operation
    for other states than 'pending'. States are obtained from service here:
    https://bodhi.fedoraproject.org/releases/<release_name>
    Unknown releases are allowed to be retired.
    
    Fixes: #337
    JIRA: COMPOSE-3690
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>