remote-add: Add --force option to add or replace remote

This uses the OSTREE_REPO_REMOTE_CHANGE_REPLACE operation to add a
remote or replace an existing one. This is roughly the opposite of
--if-not-exists and will raise an error if both options are passed.

Closes: #1166
Approved by: cgwalters

bash/ostree

@@ -985,6 +985,7 @@ _ostree_remote_add() {
     local boolean_options="
         $main_boolean_options
         --if-not-exists
+        --force
         --no-gpg-verify
     "