image_container: Fix incorrect arch processing
    
    OSBS will reject no scratch builds with arch_override.
    
    When the option is not specified in Pungi, it would do `"".split(" ")`
    to get list of arches, which returns a list with empty string instead of
    an empty list.
    
    With this fixed, it might be possible to have multiple images match the
    spec (unless arch is used in the filter). To fix that, we can replace
    arch with $basearch variable.
    
    JIRA: RHELCMP-3824
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>