Azure: Add Rawhide definitions
    
    - allow override variables template file with an externally
    provided one. This allows to add new Azure Pipeline which will
    point to a custom platform definition. Note: Azure's WebUI
    variables are runtime variables and not available at parsing time,
    that's why it's impossible to override template from WebUI in
    this case.
    
    - add Rawhide templates
    
    - add Dockerfile for build Rawhie Docker image for tests phase
    Note: 'fedora:rawhide' is too old, use for now
    'registry.fedoraproject.org/fedora:rawhide'.
    See, https://bugzilla.redhat.com/show_bug.cgi?id=1869612
    
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>