If a repo or install tree is specified as an absolute path on the local
filesystem, we should either translate it using the configured mappings,
or if no mapping matches, it should be return unchanged. A variant name
can not start with a slash, so attempting that translation does not make
much sense.
JIRA: COMPOSE-3290
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>