ccs-flatten: fix "convenient local fallback" vs. RA_METADATA_DIR

... specifically in case RA_METADATA_DIR exists but does not have the
metadata files installed (typical for rpmbuild invoked on machine
with fence-agents installed and no clufter instance yet, see also:
https://bugzilla.redhat.com/1161332)

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>