Support more specific config for devel modules
    
    The initial implementation is very coarse. It enables it for all
    variants and all modules. That is not always wanted.
    
    With this patch, the config file has to explicitly list the devel
    modules for each variant that should have it. The variant must be
    configured also to include the non-devel module (but the module may be
    in lookaside so it won't be included).
    
    We now include module metadata in the internal lookaside repo, so that
    this whole thing works if one variant is built on top of another.
    
    JIRA: COMPOSE-3034
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>