`fedpkg update`: add option `--severity`
    
    So far `--severity` was present only in a template and had to be
    uncommented in case of a security update. Now the update subcommand
    has this option with 'unspecified' as a default value.
    
    JIRA: RHELCMP-12518
    Fixes: #526
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>