init: Validate whitespace in comps groups
    
    If a package name contains leading or trailing whitespace, it will
    eventually lead to issues: pungi will try to include that group, but
    since it does not exist, the packages will not make it in.
    
    The root cause is hard to find. Better report an error immediately.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>