keymgr: Make ArtiNativeKeyStore::key_path() return a relative path.
This also updates ArtiNativeKeyStore
's KeyStore::remove
implementation to build the absolute path of the file being removed, by
joining self.keystore_dir
and the relpath returned by
ArtiNativeKeyStore::key_path()
.
This is a follow-up to !1263 (merged) (closes #908 (closed))