NCBC-664: HOST placeholder causes a UriFormatException in UpddateBootstrapList method

When a config is requested, if it contains any $HOST placeholders they
will be replaced by the address of the endpoint making the config request.
This will happen before the UpdateBootstrapList method is called; before
the config is used, because the swap occurs at the IO operation layer.

Change-Id: I85e5412d7e87e5aaf00ba5a02b91d5422d6c3e90
Reviewed-on: http://review.couchbase.org/41839
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>