backend: Handle raw config file in subdirectory implicitly
There's *path* config (see docs/raw_config.rst) which can be used
when config files in subdirectory.
With this patch ODCS can handle it implicitly/automatically if
*config_filename* prefixed with a subpath.
JIRA: RHELCMP-10818
Signed-off-by: Haibo Lin <hlin@redhat.com>