core: add "tls-ca-path" option

Some organizations will want to use private Certificate Authorities to
serve content to their clients.  While it's possible to add the CA
to the system-wide CA store, that has two drawbacks:

1) Compromise of that cert means it can be used for other web traffic
2) All of ca-certificates is trusted

This patch allows a much stronger scenario where *only* the CAs in
tls-ca-path are used for verification from the given repository.

https://bugzilla.gnome.org/show_bug.cgi?id=726256

src/libostree/ostree-fetcher.h

@@ -57,6 +57,9 @@ OstreeFetcher *_ostree_fetcher_new (GFile                     *tmpdir,
 void _ostree_fetcher_set_client_cert (OstreeFetcher *fetcher,
                                      GTlsCertificate *cert);
 
+void _ostree_fetcher_set_tls_database (OstreeFetcher *self,
+                                       GTlsDatabase *db);
+
 char * _ostree_fetcher_query_state_text (OstreeFetcher              *self);
 
 guint64 _ostree_fetcher_bytes_transferred (OstreeFetcher       *self);