Make excludedRefsPattern() to return an empty list by default

The extension of the replication endpoints configuration done
with I42366148cfb1f introduced a breaking change that made other
plugins (e.g. pull-replication) to fail.

Restore a backward compatible implementation of the new
excludedRefPattern configuration and amend the documentation to
specify that is empty by default.

Change-Id: Iee92281ed41ca70819de783b3563832363c245fb