container-build: add --isolated argument
    
    Add support for a new "--isolated" argument to the container-build
    sub-command.
    
    Isolated builds will only update the {version}-{release} unique tag and
    the primary tag in target container registry. Also, OSBS's bump_release
    plugin will ignore isolated builds.
    
    Users must specify a --build-release argument when the use the
    --isolated argument.
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>