repo: Add OstreeRepoRemoteChange replace operation

repo: Add OstreeRepoRemoteChange replace operation

Add the OSTREE_REPO_REMOTE_CHANGE_REPLACE operation to the
OstreeRepoRemoteChange enum. This operation will add a remote or replace
an existing one. It respects the location of the remote configuration
file when replacing and the remotes config dir settings when adding a
new remote.

Closes: #1166
Approved by: cgwalters